diff --git a/src/JsonApi/Serializer/ItemNormalizer.php b/src/JsonApi/Serializer/ItemNormalizer.php
index 046c0ee5310..bad4d6110ad 100644
--- a/src/JsonApi/Serializer/ItemNormalizer.php
+++ b/src/JsonApi/Serializer/ItemNormalizer.php
@@ -15,6 +15,8 @@
 
 use ApiPlatform\Metadata\ApiProperty;
 use ApiPlatform\Metadata\Exception\ItemNotFoundException;
+use ApiPlatform\Metadata\HttpOperation;
+use ApiPlatform\Metadata\IdentifiersExtractorInterface;
 use ApiPlatform\Metadata\IriConverterInterface;
 use ApiPlatform\Metadata\Property\Factory\PropertyMetadataFactoryInterface;
 use ApiPlatform\Metadata\Property\Factory\PropertyNameCollectionFactoryInterface;
@@ -23,6 +25,7 @@
 use ApiPlatform\Metadata\ResourceClassResolverInterface;
 use ApiPlatform\Metadata\UrlGeneratorInterface;
 use ApiPlatform\Metadata\Util\ClassInfoTrait;
+use ApiPlatform\Metadata\Util\CompositeIdentifierParser;
 use ApiPlatform\Metadata\Util\TypeHelper;
 use ApiPlatform\Serializer\AbstractItemNormalizer;
 use ApiPlatform\Serializer\CacheKeyTrait;
@@ -59,10 +62,26 @@ final class ItemNormalizer extends AbstractItemNormalizer
     public const FORMAT = 'jsonapi';
 
     private array $componentsCache = [];
-
-    public function __construct(PropertyNameCollectionFactoryInterface $propertyNameCollectionFactory, PropertyMetadataFactoryInterface $propertyMetadataFactory, IriConverterInterface $iriConverter, ResourceClassResolverInterface $resourceClassResolver, ?PropertyAccessorInterface $propertyAccessor = null, ?NameConverterInterface $nameConverter = null, ?ClassMetadataFactoryInterface $classMetadataFactory = null, array $defaultContext = [], ?ResourceMetadataCollectionFactoryInterface $resourceMetadataCollectionFactory = null, ?ResourceAccessCheckerInterface $resourceAccessChecker = null, protected ?TagCollectorInterface $tagCollector = null, ?OperationResourceClassResolverInterface $operationResourceResolver = null)
-    {
+    private bool $useIriAsId;
+
+    public function __construct(
+        PropertyNameCollectionFactoryInterface $propertyNameCollectionFactory,
+        PropertyMetadataFactoryInterface $propertyMetadataFactory,
+        IriConverterInterface $iriConverter,
+        ResourceClassResolverInterface $resourceClassResolver,
+        ?PropertyAccessorInterface $propertyAccessor = null,
+        ?NameConverterInterface $nameConverter = null,
+        ?ClassMetadataFactoryInterface $classMetadataFactory = null,
+        array $defaultContext = [],
+        ?ResourceMetadataCollectionFactoryInterface $resourceMetadataCollectionFactory = null,
+        ?ResourceAccessCheckerInterface $resourceAccessChecker = null,
+        protected ?TagCollectorInterface $tagCollector = null,
+        ?OperationResourceClassResolverInterface $operationResourceResolver = null,
+        private readonly ?IdentifiersExtractorInterface $identifiersExtractor = null,
+        bool $useIriAsId = true,
+    ) {
         parent::__construct($propertyNameCollectionFactory, $propertyMetadataFactory, $iriConverter, $resourceClassResolver, $propertyAccessor, $nameConverter, $classMetadataFactory, $defaultContext, $resourceMetadataCollectionFactory, $resourceAccessChecker, $tagCollector, $operationResourceResolver);
+        $this->useIriAsId = $useIriAsId;
     }
 
     /**
@@ -121,16 +140,29 @@ public function normalize(mixed $data, ?string $format = null, array $context =
         $populatedRelationContext = $context;
         $relationshipsData = $this->getPopulatedRelations($data, $format, $populatedRelationContext, $allRelationshipsData);
 
-        // Do not include primary resources
-        $context['api_included_resources'] = [$context['iri']];
+        $id = $iri;
+        if (!$this->useIriAsId) {
+            $identifiers = $this->identifiersExtractor->getIdentifiersFromItem($data, context: $context);
+            $id = $this->getIdStringFromIdentifiers($identifiers);
+        }
+
+        $resourceShortName = $this->getResourceShortName($resourceClass);
+
+        // Do not include primary resources — use type:id composite key to avoid cross-type collisions
+        $context['api_included_resources'] = [$resourceShortName.':'.$id => true];
 
         $includedResourcesData = $this->getRelatedResources($data, $format, $context, $allRelationshipsData);
 
         $resourceData = [
-            'id' => $context['iri'],
-            'type' => $this->getResourceShortName($resourceClass),
+            'id' => $id,
+            'type' => $resourceShortName,
         ];
 
+        // TODO: consider always adding links.self — it's valid per the JSON:API spec even when id is the IRI
+        if (!$this->useIriAsId) {
+            $resourceData['links'] = ['self' => $iri];
+        }
+
         if ($normalizedData) {
             $resourceData['attributes'] = $normalizedData;
         }
@@ -175,10 +207,19 @@ public function denormalize(mixed $data, string $type, ?string $format = null, a
                 throw new NotNormalizableValueException('Update is not allowed for this operation.');
             }
 
-            $context[self::OBJECT_TO_POPULATE] = $this->iriConverter->getResourceFromIri(
-                $data['data']['id'],
-                $context + ['fetch_data' => false]
-            );
+            $context += ['fetch_data' => false];
+            if ($this->useIriAsId) {
+                $context[self::OBJECT_TO_POPULATE] = $this->iriConverter->getResourceFromIri(
+                    $data['data']['id'],
+                    $context
+                );
+            } else {
+                $operation = $context['operation'] ?? null;
+                if ($operation instanceof HttpOperation) {
+                    $iri = $this->reconstructIri($type, (string) $data['data']['id'], $operation);
+                    $context[self::OBJECT_TO_POPULATE] = $this->iriConverter->getResourceFromIri($iri, $context);
+                }
+            }
         }
 
         // Merge attributes and relationships, into format expected by the parent normalizer
@@ -226,7 +267,29 @@ protected function denormalizeRelation(string $attributeName, ApiProperty $prope
         }
 
         try {
-            return $this->iriConverter->getResourceFromIri($value['id'], $context + ['fetch_data' => true]);
+            $context += ['fetch_data' => true];
+            if ($this->useIriAsId) {
+                return $this->iriConverter->getResourceFromIri($value['id'], $context);
+            }
+
+            $targetClass = null;
+            $nativeType = $propertyMetadata->getNativeType();
+
+            if ($nativeType) {
+                $nativeType->isSatisfiedBy(function (Type $type) use (&$targetClass): bool {
+                    return $type instanceof ObjectType && $this->resourceClassResolver->isResourceClass($targetClass = $type->getClassName());
+                });
+            }
+
+            if (null === $targetClass) {
+                throw new ItemNotFoundException(\sprintf('Cannot determine target class for property "%s".', $attributeName));
+            }
+
+            /** @var HttpOperation $getOperation */
+            $getOperation = $this->resourceMetadataCollectionFactory->create($targetClass)->getOperation(httpOperation: true);
+            $iri = $this->reconstructIri($targetClass, (string) $value['id'], $getOperation);
+
+            return $this->iriConverter->getResourceFromIri($iri, $context);
         } catch (ItemNotFoundException $e) {
             if (!isset($context['not_normalizable_value_exceptions'])) {
                 throw new RuntimeException($e->getMessage(), $e->getCode(), $e);
@@ -274,11 +337,19 @@ protected function normalizeRelation(ApiProperty $propertyMetadata, ?object $rel
             return $normalizedRelatedObject;
         }
 
+        $id = $iri;
+        if (!$this->useIriAsId) {
+            $identifiers = $this->identifiersExtractor->getIdentifiersFromItem($relatedObject);
+            $id = $this->getIdStringFromIdentifiers($identifiers);
+        }
+
+        $relationData = [
+            'type' => $this->getResourceShortName($resourceClass),
+            'id' => $id,
+        ];
+
         $context['data'] = [
-            'data' => [
-                'type' => $this->getResourceShortName($resourceClass),
-                'id' => $iri,
-            ],
+            'data' => $relationData,
         ];
 
         $context['iri'] = $iri;
@@ -551,10 +622,10 @@ private function getRelatedResources(object $object, ?string $format, array $con
      */
     private function addIncluded(array $data, array &$included, array &$context): void
     {
-        if (isset($data['id']) && !\in_array($data['id'], $context['api_included_resources'], true)) {
+        $trackingKey = ($data['type'] ?? '').':'.($data['id'] ?? '');
+        if (isset($data['id']) && !isset($context['api_included_resources'][$trackingKey])) {
             $included[] = $data;
-            // Track already included resources
-            $context['api_included_resources'][] = $data['id'];
+            $context['api_included_resources'][$trackingKey] = true;
         }
     }
 
@@ -580,6 +651,35 @@ private function getIncludedNestedResources(string $relationshipName, array $con
         return array_map(static fn (string $nested): string => substr($nested, strpos($nested, '.') + 1), $filtered);
     }
 
+    private function getIdStringFromIdentifiers(array $identifiers): string
+    {
+        if (1 === \count($identifiers)) {
+            return (string) array_values($identifiers)[0];
+        }
+
+        return CompositeIdentifierParser::stringify($identifiers);
+    }
+
+    /**
+     * Reconstructs an IRI from a resource class and a raw JSON:API id string.
+     *
+     * Maps the id to the operation's single URI variable parameter name and generates
+     * the IRI via IriConverter. Composite identifiers on a single Link work naturally
+     * since the composite string (e.g. "field1=val1;field2=val2") is passed as-is.
+     */
+    private function reconstructIri(string $resourceClass, string $id, HttpOperation $operation): string
+    {
+        $uriVariables = $operation->getUriVariables() ?? [];
+
+        if (\count($uriVariables) > 1) {
+            throw new UnexpectedValueException(\sprintf('JSON:API entity identifier mode requires operations with a single URI variable, operation "%s" has %d. Consider adding a NotExposed Get operation on the resource.', $operation->getName() ?? $operation->getUriTemplate(), \count($uriVariables)));
+        }
+
+        $parameterName = array_key_first($uriVariables) ?? 'id';
+
+        return $this->iriConverter->getIriFromResource($resourceClass, UrlGeneratorInterface::ABS_PATH, $operation, ['uri_variables' => [$parameterName => $id]]);
+    }
+
     // TODO: this code is similar to the one used in JsonLd
     private function getResourceShortName(string $resourceClass): string
     {
diff --git a/src/JsonApi/Tests/Serializer/ItemNormalizerTest.php b/src/JsonApi/Tests/Serializer/ItemNormalizerTest.php
index a9af4cf4b5d..a11d4d2b8f1 100644
--- a/src/JsonApi/Tests/Serializer/ItemNormalizerTest.php
+++ b/src/JsonApi/Tests/Serializer/ItemNormalizerTest.php
@@ -21,7 +21,9 @@
 use ApiPlatform\Metadata\ApiProperty;
 use ApiPlatform\Metadata\ApiResource;
 use ApiPlatform\Metadata\Get;
+use ApiPlatform\Metadata\IdentifiersExtractorInterface;
 use ApiPlatform\Metadata\IriConverterInterface;
+use ApiPlatform\Metadata\Link;
 use ApiPlatform\Metadata\Operations;
 use ApiPlatform\Metadata\Property\Factory\PropertyMetadataFactoryInterface;
 use ApiPlatform\Metadata\Property\Factory\PropertyNameCollectionFactoryInterface;
@@ -29,6 +31,7 @@
 use ApiPlatform\Metadata\Resource\Factory\ResourceMetadataCollectionFactoryInterface;
 use ApiPlatform\Metadata\Resource\ResourceMetadataCollection;
 use ApiPlatform\Metadata\ResourceClassResolverInterface;
+use ApiPlatform\Metadata\UrlGeneratorInterface;
 use Doctrine\Common\Collections\ArrayCollection;
 use PHPUnit\Framework\TestCase;
 use Prophecy\Argument;
@@ -577,4 +580,328 @@ public function testNormalizeWithNullToOneAndEmptyToManyRelationships(): void
         $this->assertArrayHasKey('relatedDummies', $result['data']['relationships']);
         $this->assertSame(['data' => []], $result['data']['relationships']['relatedDummies']);
     }
+
+    public function testNormalizeWithEntityIdentifier(): void
+    {
+        $dummy = new Dummy();
+        $dummy->setId(10);
+        $dummy->setName('hello');
+
+        $propertyNameCollectionFactoryProphecy = $this->prophesize(PropertyNameCollectionFactoryInterface::class);
+        $propertyNameCollectionFactoryProphecy->create(Dummy::class, Argument::any())->willReturn(new PropertyNameCollection(['id', 'name']));
+
+        $propertyMetadataFactoryProphecy = $this->prophesize(PropertyMetadataFactoryInterface::class);
+        $propertyMetadataFactoryProphecy->create(Dummy::class, 'name', Argument::any())->willReturn((new ApiProperty())->withReadable(true));
+        $propertyMetadataFactoryProphecy->create(Dummy::class, 'id', Argument::type('array'))->willReturn((new ApiProperty())->withReadable(true)->withIdentifier(true));
+
+        $iriConverterProphecy = $this->prophesize(IriConverterInterface::class);
+        $iriConverterProphecy->getIriFromResource($dummy, Argument::cetera())->willReturn('/dummies/10');
+
+        $resourceClassResolverProphecy = $this->prophesize(ResourceClassResolverInterface::class);
+        $resourceClassResolverProphecy->getResourceClass($dummy, null)->willReturn(Dummy::class);
+        $resourceClassResolverProphecy->getResourceClass(null, Dummy::class)->willReturn(Dummy::class);
+        $resourceClassResolverProphecy->getResourceClass($dummy, Dummy::class)->willReturn(Dummy::class);
+        $resourceClassResolverProphecy->isResourceClass(Dummy::class)->willReturn(true);
+
+        $propertyAccessorProphecy = $this->prophesize(PropertyAccessorInterface::class);
+        $propertyAccessorProphecy->getValue($dummy, 'id')->willReturn(10);
+        $propertyAccessorProphecy->getValue($dummy, 'name')->willReturn('hello');
+
+        $resourceMetadataCollectionFactoryProphecy = $this->prophesize(ResourceMetadataCollectionFactoryInterface::class);
+        $resourceMetadataCollectionFactoryProphecy->create(Dummy::class)->willReturn(new ResourceMetadataCollection('Dummy', [
+            (new ApiResource())
+                ->withShortName('Dummy')
+                ->withOperations(new Operations(['get' => (new Get())->withShortName('Dummy')])),
+        ]));
+
+        $identifiersExtractorProphecy = $this->prophesize(IdentifiersExtractorInterface::class);
+        $identifiersExtractorProphecy->getIdentifiersFromItem($dummy, Argument::any(), Argument::type('array'))->willReturn(['id' => 10]);
+
+        $serializerProphecy = $this->prophesize(SerializerInterface::class);
+        $serializerProphecy->willImplement(NormalizerInterface::class);
+        $serializerProphecy->normalize('hello', ItemNormalizer::FORMAT, Argument::type('array'))->willReturn('hello');
+        $serializerProphecy->normalize(10, ItemNormalizer::FORMAT, Argument::type('array'))->willReturn(10);
+        $serializerProphecy->normalize(null, ItemNormalizer::FORMAT, Argument::type('array'))->willReturn(null);
+
+        $normalizer = new ItemNormalizer(
+            $propertyNameCollectionFactoryProphecy->reveal(),
+            $propertyMetadataFactoryProphecy->reveal(),
+            $iriConverterProphecy->reveal(),
+            $resourceClassResolverProphecy->reveal(),
+            $propertyAccessorProphecy->reveal(),
+            new ReservedAttributeNameConverter(),
+            null,
+            [],
+            $resourceMetadataCollectionFactoryProphecy->reveal(),
+            null,
+            null,
+            null,
+            $identifiersExtractorProphecy->reveal(),
+            false,
+        );
+
+        $normalizer->setSerializer($serializerProphecy->reveal());
+
+        $expected = [
+            'data' => [
+                'id' => '10',
+                'type' => 'Dummy',
+                'links' => ['self' => '/dummies/10'],
+                'attributes' => [
+                    '_id' => 10,
+                    'name' => 'hello',
+                ],
+            ],
+        ];
+
+        $this->assertEquals($expected, $normalizer->normalize($dummy, ItemNormalizer::FORMAT));
+    }
+
+    public function testNormalizeRelationWithEntityIdentifier(): void
+    {
+        $dummy = new Dummy();
+        $dummy->setId(10);
+        $dummy->setName('hello');
+
+        $relatedDummy = new RelatedDummy();
+        $relatedDummy->setId(1);
+
+        $propertyNameCollectionFactoryProphecy = $this->prophesize(PropertyNameCollectionFactoryInterface::class);
+        $propertyNameCollectionFactoryProphecy->create(Dummy::class, Argument::any())->willReturn(new PropertyNameCollection(['id', 'name', 'relatedDummy']));
+
+        $relatedDummyType = Type::object(RelatedDummy::class);
+
+        $propertyMetadataFactoryProphecy = $this->prophesize(PropertyMetadataFactoryInterface::class);
+        $propertyMetadataFactoryProphecy->create(Dummy::class, 'name', Argument::any())->willReturn((new ApiProperty())->withReadable(true));
+        $propertyMetadataFactoryProphecy->create(Dummy::class, 'id', Argument::type('array'))->willReturn((new ApiProperty())->withReadable(true)->withIdentifier(true));
+        $propertyMetadataFactoryProphecy->create(Dummy::class, 'relatedDummy', Argument::any())->willReturn(
+            (new ApiProperty())->withNativeType($relatedDummyType)->withReadable(true)->withReadableLink(false)->withWritableLink(false)
+        );
+
+        $iriConverterProphecy = $this->prophesize(IriConverterInterface::class);
+        $iriConverterProphecy->getIriFromResource($dummy, Argument::cetera())->willReturn('/dummies/10');
+        $iriConverterProphecy->getIriFromResource($relatedDummy)->willReturn('/related_dummies/1');
+
+        $resourceClassResolverProphecy = $this->prophesize(ResourceClassResolverInterface::class);
+        $resourceClassResolverProphecy->getResourceClass($dummy, null)->willReturn(Dummy::class);
+        $resourceClassResolverProphecy->getResourceClass(null, Dummy::class)->willReturn(Dummy::class);
+        $resourceClassResolverProphecy->getResourceClass($dummy, Dummy::class)->willReturn(Dummy::class);
+        $resourceClassResolverProphecy->getResourceClass($relatedDummy, RelatedDummy::class)->willReturn(RelatedDummy::class);
+        $resourceClassResolverProphecy->isResourceClass(Dummy::class)->willReturn(true);
+        $resourceClassResolverProphecy->isResourceClass(RelatedDummy::class)->willReturn(true);
+
+        $propertyAccessorProphecy = $this->prophesize(PropertyAccessorInterface::class);
+        $propertyAccessorProphecy->getValue($dummy, 'id')->willReturn(10);
+        $propertyAccessorProphecy->getValue($dummy, 'name')->willReturn('hello');
+        $propertyAccessorProphecy->getValue($dummy, 'relatedDummy')->willReturn($relatedDummy);
+
+        $resourceMetadataCollectionFactoryProphecy = $this->prophesize(ResourceMetadataCollectionFactoryInterface::class);
+        $resourceMetadataCollectionFactoryProphecy->create(Dummy::class)->willReturn(new ResourceMetadataCollection('Dummy', [
+            (new ApiResource())
+                ->withShortName('Dummy')
+                ->withOperations(new Operations(['get' => (new Get())->withShortName('Dummy')])),
+        ]));
+        $resourceMetadataCollectionFactoryProphecy->create(RelatedDummy::class)->willReturn(new ResourceMetadataCollection('RelatedDummy', [
+            (new ApiResource())
+                ->withShortName('RelatedDummy')
+                ->withOperations(new Operations(['get' => (new Get())->withShortName('RelatedDummy')])),
+        ]));
+
+        $identifiersExtractorProphecy = $this->prophesize(IdentifiersExtractorInterface::class);
+        $identifiersExtractorProphecy->getIdentifiersFromItem($dummy, Argument::any(), Argument::type('array'))->willReturn(['id' => 10]);
+        $identifiersExtractorProphecy->getIdentifiersFromItem($relatedDummy)->willReturn(['id' => 1]);
+
+        $serializerProphecy = $this->prophesize(SerializerInterface::class);
+        $serializerProphecy->willImplement(NormalizerInterface::class);
+        $serializerProphecy->normalize('hello', ItemNormalizer::FORMAT, Argument::type('array'))->willReturn('hello');
+        $serializerProphecy->normalize(10, ItemNormalizer::FORMAT, Argument::type('array'))->willReturn(10);
+        $serializerProphecy->normalize(null, ItemNormalizer::FORMAT, Argument::type('array'))->willReturn(null);
+
+        $normalizer = new ItemNormalizer(
+            $propertyNameCollectionFactoryProphecy->reveal(),
+            $propertyMetadataFactoryProphecy->reveal(),
+            $iriConverterProphecy->reveal(),
+            $resourceClassResolverProphecy->reveal(),
+            $propertyAccessorProphecy->reveal(),
+            new ReservedAttributeNameConverter(),
+            null,
+            [],
+            $resourceMetadataCollectionFactoryProphecy->reveal(),
+            null,
+            null,
+            null,
+            $identifiersExtractorProphecy->reveal(),
+            false,
+        );
+
+        $normalizer->setSerializer($serializerProphecy->reveal());
+
+        $result = $normalizer->normalize($dummy, ItemNormalizer::FORMAT, [
+            'resources' => [],
+            'resource_class' => Dummy::class,
+        ]);
+
+        $this->assertIsArray($result);
+        $this->assertSame('10', $result['data']['id']);
+        $this->assertSame('Dummy', $result['data']['type']);
+        $this->assertSame('/dummies/10', $result['data']['links']['self']);
+
+        // Verify relationship uses entity identifier (no links — resource identifier objects are spec-compliant)
+        $this->assertArrayHasKey('relationships', $result['data']);
+        $this->assertArrayHasKey('relatedDummy', $result['data']['relationships']);
+        $relationData = $result['data']['relationships']['relatedDummy']['data'];
+        $this->assertSame('1', $relationData['id']);
+        $this->assertSame('RelatedDummy', $relationData['type']);
+        $this->assertArrayNotHasKey('links', $relationData);
+    }
+
+    public function testDenormalizeWithEntityIdentifier(): void
+    {
+        $data = [
+            'data' => [
+                'type' => 'dummy',
+                'id' => '10',
+                'attributes' => [
+                    'name' => 'foo',
+                ],
+            ],
+        ];
+
+        $dummy = new Dummy();
+        $dummy->setId(10);
+
+        $propertyNameCollectionFactoryProphecy = $this->prophesize(PropertyNameCollectionFactoryInterface::class);
+        $propertyNameCollectionFactoryProphecy->create(Dummy::class, Argument::any())->willReturn(new PropertyNameCollection(['name']));
+
+        $propertyMetadataFactoryProphecy = $this->prophesize(PropertyMetadataFactoryInterface::class);
+        $propertyMetadataFactoryProphecy->create(Dummy::class, 'name', Argument::any())->willReturn((new ApiProperty())->withNativeType(Type::string())->withReadable(false)->withWritable(true));
+
+        $getOperation = (new Get(uriVariables: ['id' => new Link(parameterName: 'id', identifiers: ['id'], fromClass: Dummy::class)]))->withClass(Dummy::class);
+
+        $iriConverterProphecy = $this->prophesize(IriConverterInterface::class);
+        $iriConverterProphecy->getIriFromResource(Dummy::class, UrlGeneratorInterface::ABS_PATH, $getOperation, Argument::that(static fn ($ctx) => ($ctx['uri_variables'] ?? null) === ['id' => '10']))->willReturn('/dummies/10');
+        $iriConverterProphecy->getResourceFromIri('/dummies/10', Argument::type('array'))->willReturn($dummy);
+
+        $propertyAccessorProphecy = $this->prophesize(PropertyAccessorInterface::class);
+        $propertyAccessorProphecy->setValue(Argument::type(Dummy::class), 'name', 'foo')->will(static function (): void {});
+
+        $resourceClassResolverProphecy = $this->prophesize(ResourceClassResolverInterface::class);
+        $resourceClassResolverProphecy->getResourceClass(null, Dummy::class)->willReturn(Dummy::class);
+        $resourceClassResolverProphecy->getResourceClass($dummy, Dummy::class)->willReturn(Dummy::class);
+        $resourceClassResolverProphecy->isResourceClass(Dummy::class)->willReturn(true);
+
+        $resourceMetadataCollectionFactory = $this->prophesize(ResourceMetadataCollectionFactoryInterface::class);
+        $resourceMetadataCollectionFactory->create(Dummy::class)->willReturn(new ResourceMetadataCollection(Dummy::class, [
+            (new ApiResource())->withOperations(new Operations(['get' => $getOperation])),
+        ]));
+
+        $normalizer = new ItemNormalizer(
+            $propertyNameCollectionFactoryProphecy->reveal(),
+            $propertyMetadataFactoryProphecy->reveal(),
+            $iriConverterProphecy->reveal(),
+            $resourceClassResolverProphecy->reveal(),
+            $propertyAccessorProphecy->reveal(),
+            new ReservedAttributeNameConverter(),
+            null,
+            [],
+            $resourceMetadataCollectionFactory->reveal(),
+            null,
+            null,
+            null,
+            null,
+            false,
+        );
+
+        $serializerProphecy = $this->prophesize(SerializerInterface::class);
+        $serializerProphecy->willImplement(NormalizerInterface::class);
+        $normalizer->setSerializer($serializerProphecy->reveal());
+
+        $result = $normalizer->denormalize($data, Dummy::class, ItemNormalizer::FORMAT, [
+            'operation' => $getOperation,
+        ]);
+
+        $this->assertInstanceOf(Dummy::class, $result);
+    }
+
+    public function testDenormalizeRelationWithEntityIdentifier(): void
+    {
+        $data = [
+            'data' => [
+                'type' => 'dummy',
+                'attributes' => [
+                    'name' => 'foo',
+                ],
+                'relationships' => [
+                    'relatedDummy' => [
+                        'data' => [
+                            'type' => 'related-dummy',
+                            'id' => '1',
+                        ],
+                    ],
+                ],
+            ],
+        ];
+
+        $relatedDummy = new RelatedDummy();
+        $relatedDummy->setId(1);
+
+        $relatedDummyType = Type::object(RelatedDummy::class);
+        $getRelatedOperation = (new Get(uriVariables: ['id' => new Link(parameterName: 'id', identifiers: ['id'], fromClass: RelatedDummy::class)]))->withClass(RelatedDummy::class);
+
+        $propertyNameCollectionFactoryProphecy = $this->prophesize(PropertyNameCollectionFactoryInterface::class);
+        $propertyNameCollectionFactoryProphecy->create(Dummy::class, Argument::any())->willReturn(new PropertyNameCollection(['name', 'relatedDummy']));
+
+        $propertyMetadataFactoryProphecy = $this->prophesize(PropertyMetadataFactoryInterface::class);
+        $propertyMetadataFactoryProphecy->create(Dummy::class, 'name', Argument::any())->willReturn((new ApiProperty())->withNativeType(Type::string())->withReadable(false)->withWritable(true));
+        $propertyMetadataFactoryProphecy->create(Dummy::class, 'relatedDummy', Argument::any())->willReturn(
+            (new ApiProperty())->withNativeType($relatedDummyType)->withReadable(false)->withWritable(true)->withReadableLink(false)->withWritableLink(false)
+        );
+
+        $iriConverterProphecy = $this->prophesize(IriConverterInterface::class);
+        $iriConverterProphecy->getIriFromResource(RelatedDummy::class, UrlGeneratorInterface::ABS_PATH, $getRelatedOperation, Argument::that(static fn ($ctx) => ($ctx['uri_variables'] ?? null) === ['id' => '1']))->willReturn('/related_dummies/1');
+        $iriConverterProphecy->getResourceFromIri('/related_dummies/1', Argument::type('array'))->willReturn($relatedDummy);
+
+        $propertyAccessorProphecy = $this->prophesize(PropertyAccessorInterface::class);
+        $propertyAccessorProphecy->setValue(Argument::type(Dummy::class), 'name', 'foo')->will(static function (): void {});
+        $propertyAccessorProphecy->setValue(Argument::type(Dummy::class), 'relatedDummy', $relatedDummy)->will(static function (): void {});
+
+        $resourceClassResolverProphecy = $this->prophesize(ResourceClassResolverInterface::class);
+        $resourceClassResolverProphecy->getResourceClass(null, Dummy::class)->willReturn(Dummy::class);
+        $resourceClassResolverProphecy->getResourceClass(null, RelatedDummy::class)->willReturn(RelatedDummy::class);
+        $resourceClassResolverProphecy->isResourceClass(Dummy::class)->willReturn(true);
+        $resourceClassResolverProphecy->isResourceClass(RelatedDummy::class)->willReturn(true);
+
+        $resourceMetadataCollectionFactory = $this->prophesize(ResourceMetadataCollectionFactoryInterface::class);
+        $resourceMetadataCollectionFactory->create(Dummy::class)->willReturn(new ResourceMetadataCollection(Dummy::class, [
+            (new ApiResource())->withOperations(new Operations([new Get(name: 'get')])),
+        ]));
+        $resourceMetadataCollectionFactory->create(RelatedDummy::class)->willReturn(new ResourceMetadataCollection(RelatedDummy::class, [
+            (new ApiResource())->withOperations(new Operations(['get' => $getRelatedOperation])),
+        ]));
+
+        $normalizer = new ItemNormalizer(
+            $propertyNameCollectionFactoryProphecy->reveal(),
+            $propertyMetadataFactoryProphecy->reveal(),
+            $iriConverterProphecy->reveal(),
+            $resourceClassResolverProphecy->reveal(),
+            $propertyAccessorProphecy->reveal(),
+            new ReservedAttributeNameConverter(),
+            null,
+            [],
+            $resourceMetadataCollectionFactory->reveal(),
+            null,
+            null,
+            null,
+            null,
+            false,
+        );
+
+        $serializerProphecy = $this->prophesize(SerializerInterface::class);
+        $serializerProphecy->willImplement(NormalizerInterface::class);
+        $normalizer->setSerializer($serializerProphecy->reveal());
+
+        $result = $normalizer->denormalize($data, Dummy::class, ItemNormalizer::FORMAT);
+
+        $this->assertInstanceOf(Dummy::class, $result);
+    }
 }
diff --git a/src/Symfony/Bundle/DependencyInjection/ApiPlatformExtension.php b/src/Symfony/Bundle/DependencyInjection/ApiPlatformExtension.php
index 1f2f6689b96..42b625cf8ee 100644
--- a/src/Symfony/Bundle/DependencyInjection/ApiPlatformExtension.php
+++ b/src/Symfony/Bundle/DependencyInjection/ApiPlatformExtension.php
@@ -176,7 +176,7 @@ public function load(array $configs, ContainerBuilder $container): void
         $this->registerOAuthConfiguration($container, $config);
         $this->registerOpenApiConfiguration($container, $config, $loader);
         $this->registerSwaggerConfiguration($container, $config, $loader);
-        $this->registerJsonApiConfiguration($formats, $loader, $config);
+        $this->registerJsonApiConfiguration($container, $formats, $loader, $config);
         $this->registerJsonLdHydraConfiguration($container, $formats, $loader, $config);
         $this->registerJsonHalConfiguration($formats, $loader);
         $this->registerJsonProblemConfiguration($errorFormats, $loader);
@@ -676,7 +676,7 @@ private function registerSwaggerConfiguration(ContainerBuilder $container, array
         $container->setParameter('api_platform.swagger_ui.extra_configuration', $config['openapi']['swagger_ui_extra_configuration'] ?: $config['swagger']['swagger_ui_extra_configuration']);
     }
 
-    private function registerJsonApiConfiguration(array $formats, PhpFileLoader $loader, array $config): void
+    private function registerJsonApiConfiguration(ContainerBuilder $container, array $formats, PhpFileLoader $loader, array $config): void
     {
         if (!isset($formats['jsonapi'])) {
             return;
@@ -688,6 +688,9 @@ private function registerJsonApiConfiguration(array $formats, PhpFileLoader $loa
 
         $loader->load('jsonapi.php');
         $loader->load('state/jsonapi.php');
+
+        $container->getDefinition('api_platform.jsonapi.normalizer.item')
+            ->addArgument($config['jsonapi']['use_iri_as_id']);
     }
 
     private function registerJsonLdHydraConfiguration(ContainerBuilder $container, array $formats, PhpFileLoader $loader, array $config): void
diff --git a/src/Symfony/Bundle/DependencyInjection/Configuration.php b/src/Symfony/Bundle/DependencyInjection/Configuration.php
index f97df37ad5d..43954615e50 100644
--- a/src/Symfony/Bundle/DependencyInjection/Configuration.php
+++ b/src/Symfony/Bundle/DependencyInjection/Configuration.php
@@ -99,6 +99,16 @@ public function getConfigTreeBuilder(): TreeBuilder
                         ->end()
                     ->end()
                 ->end()
+                // TODO 4.4: deprecate use_iri_as_id defaulting to true
+                ->arrayNode('jsonapi')
+                    ->addDefaultsIfNotSet()
+                        ->children()
+                        ->booleanNode('use_iri_as_id')
+                            ->defaultTrue()
+                            ->info('Set to false to use entity identifiers instead of IRIs as the "id" field in JSON:API responses.')
+                        ->end()
+                    ->end()
+                ->end()
                 ->arrayNode('eager_loading')
                     ->canBeDisabled()
                     ->addDefaultsIfNotSet()
diff --git a/src/Symfony/Bundle/Resources/config/jsonapi.php b/src/Symfony/Bundle/Resources/config/jsonapi.php
index c4e7c895bb7..6ad6d49ab4c 100644
--- a/src/Symfony/Bundle/Resources/config/jsonapi.php
+++ b/src/Symfony/Bundle/Resources/config/jsonapi.php
@@ -73,6 +73,7 @@
             service('api_platform.security.resource_access_checker')->ignoreOnInvalid(),
             service('api_platform.http_cache.tag_collector')->ignoreOnInvalid(),
             service('api_platform.serializer.operation_resource_resolver'),
+            service('api_platform.api.identifiers_extractor'),
         ])
         ->tag('serializer.normalizer', ['priority' => -890]);
 
diff --git a/tests/Fixtures/TestBundle/ApiResource/JsonApiDummy.php b/tests/Fixtures/TestBundle/ApiResource/JsonApiDummy.php
new file mode 100644
index 00000000000..10f930bb7e7
--- /dev/null
+++ b/tests/Fixtures/TestBundle/ApiResource/JsonApiDummy.php
@@ -0,0 +1,62 @@
+<?php
+
+/*
+ * This file is part of the API Platform project.
+ *
+ * (c) Kévin Dunglas <dunglas@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+declare(strict_types=1);
+
+namespace ApiPlatform\Tests\Fixtures\TestBundle\ApiResource;
+
+use ApiPlatform\Metadata\Get;
+use ApiPlatform\Metadata\GetCollection;
+use ApiPlatform\Metadata\Operation;
+
+#[Get(
+    uriTemplate: '/jsonapi_dummies/{id}',
+    formats: ['jsonapi' => ['application/vnd.api+json']],
+    provider: [self::class, 'provide'],
+)]
+#[GetCollection(
+    uriTemplate: '/jsonapi_dummies',
+    formats: ['jsonapi' => ['application/vnd.api+json']],
+    provider: [self::class, 'provideCollection'],
+)]
+class JsonApiDummy
+{
+    public function __construct(
+        public int $id = 0,
+        public string $name = '',
+        public ?JsonApiRelatedDummy $relatedDummy = null,
+        public ?JsonApiNotExposedRelation $notExposedRelation = null,
+    ) {
+    }
+
+    public static function provide(Operation $operation, array $uriVariables = [], array $context = []): self
+    {
+        $id = (int) ($uriVariables['id'] ?? 0);
+
+        return new self(
+            id: $id,
+            name: 'Dummy #'.$id,
+            relatedDummy: new JsonApiRelatedDummy(id: 1, title: 'Related #1'),
+            notExposedRelation: new JsonApiNotExposedRelation(id: 5, label: 'NotExposed #5'),
+        );
+    }
+
+    /**
+     * @return list<self>
+     */
+    public static function provideCollection(Operation $operation, array $uriVariables = [], array $context = []): array
+    {
+        return [
+            new self(id: 1, name: 'Dummy #1', relatedDummy: new JsonApiRelatedDummy(id: 1, title: 'Related #1')),
+            new self(id: 2, name: 'Dummy #2'),
+        ];
+    }
+}
diff --git a/tests/Fixtures/TestBundle/ApiResource/JsonApiNotExposedRelation.php b/tests/Fixtures/TestBundle/ApiResource/JsonApiNotExposedRelation.php
new file mode 100644
index 00000000000..c793b07c25d
--- /dev/null
+++ b/tests/Fixtures/TestBundle/ApiResource/JsonApiNotExposedRelation.php
@@ -0,0 +1,60 @@
+<?php
+
+/*
+ * This file is part of the API Platform project.
+ *
+ * (c) Kévin Dunglas <dunglas@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+declare(strict_types=1);
+
+namespace ApiPlatform\Tests\Fixtures\TestBundle\ApiResource;
+
+use ApiPlatform\Metadata\Get;
+use ApiPlatform\Metadata\Link;
+use ApiPlatform\Metadata\NotExposed;
+use ApiPlatform\Metadata\Operation;
+
+/**
+ * A resource with no public item operation — only a NotExposed GET.
+ * Also exposed as a subresource of JsonApiDummy to verify that links.self
+ * in entity identifier mode uses the NotExposed IRI, not the subresource URI.
+ */
+#[NotExposed(
+    uriTemplate: '/jsonapi_not_exposed_relations/{id}',
+    formats: ['jsonapi' => ['application/vnd.api+json']],
+    provider: [self::class, 'provide'],
+)]
+#[Get(
+    uriTemplate: '/jsonapi_dummies/{dummyId}/not_exposed_relation',
+    uriVariables: [
+        'dummyId' => new Link(fromClass: JsonApiDummy::class, fromProperty: 'notExposedRelation'),
+    ],
+    formats: ['jsonapi' => ['application/vnd.api+json']],
+    provider: [self::class, 'provide'],
+)]
+class JsonApiNotExposedRelation
+{
+    public function __construct(
+        public int $id = 0,
+        public string $label = '',
+    ) {
+    }
+
+    public static function provide(Operation $operation, array $uriVariables = [], array $context = []): self
+    {
+        if (isset($uriVariables['dummyId'])) {
+            // Subresource: resolve from parent dummy
+            $dummy = JsonApiDummy::provide($operation, ['id' => $uriVariables['dummyId']], $context);
+
+            return $dummy->notExposedRelation ?? new self();
+        }
+
+        $id = (int) ($uriVariables['id'] ?? 0);
+
+        return new self(id: $id, label: 'NotExposed #'.$id);
+    }
+}
diff --git a/tests/Fixtures/TestBundle/ApiResource/JsonApiRelatedDummy.php b/tests/Fixtures/TestBundle/ApiResource/JsonApiRelatedDummy.php
new file mode 100644
index 00000000000..54c900f08b7
--- /dev/null
+++ b/tests/Fixtures/TestBundle/ApiResource/JsonApiRelatedDummy.php
@@ -0,0 +1,38 @@
+<?php
+
+/*
+ * This file is part of the API Platform project.
+ *
+ * (c) Kévin Dunglas <dunglas@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+declare(strict_types=1);
+
+namespace ApiPlatform\Tests\Fixtures\TestBundle\ApiResource;
+
+use ApiPlatform\Metadata\Get;
+use ApiPlatform\Metadata\Operation;
+
+#[Get(
+    uriTemplate: '/jsonapi_related_dummies/{id}',
+    formats: ['jsonapi' => ['application/vnd.api+json']],
+    provider: [self::class, 'provide'],
+)]
+class JsonApiRelatedDummy
+{
+    public function __construct(
+        public int $id = 0,
+        public string $title = '',
+    ) {
+    }
+
+    public static function provide(Operation $operation, array $uriVariables = [], array $context = []): self
+    {
+        $id = (int) ($uriVariables['id'] ?? 0);
+
+        return new self(id: $id, title: 'Related #'.$id);
+    }
+}
diff --git a/tests/Fixtures/app/config/config_jsonapi.yml b/tests/Fixtures/app/config/config_jsonapi.yml
new file mode 100644
index 00000000000..16eab860316
--- /dev/null
+++ b/tests/Fixtures/app/config/config_jsonapi.yml
@@ -0,0 +1,6 @@
+imports:
+    - { resource: config_test.yml }
+
+api_platform:
+    jsonapi:
+        use_iri_as_id: false
diff --git a/tests/Fixtures/app/config/config_jsonapi_mongodb.yml b/tests/Fixtures/app/config/config_jsonapi_mongodb.yml
new file mode 100644
index 00000000000..e720a07a5fb
--- /dev/null
+++ b/tests/Fixtures/app/config/config_jsonapi_mongodb.yml
@@ -0,0 +1,6 @@
+imports:
+    - { resource: config_mongodb.yml }
+
+api_platform:
+    jsonapi:
+        use_iri_as_id: false
diff --git a/tests/Fixtures/app/config/routing_jsonapi.yml b/tests/Fixtures/app/config/routing_jsonapi.yml
new file mode 100644
index 00000000000..8f00918cd14
--- /dev/null
+++ b/tests/Fixtures/app/config/routing_jsonapi.yml
@@ -0,0 +1,2 @@
+_main:
+    resource: routing_test.php
diff --git a/tests/Fixtures/app/config/routing_jsonapi_mongodb.yml b/tests/Fixtures/app/config/routing_jsonapi_mongodb.yml
new file mode 100644
index 00000000000..691266c01db
--- /dev/null
+++ b/tests/Fixtures/app/config/routing_jsonapi_mongodb.yml
@@ -0,0 +1,2 @@
+_main:
+    resource: routing_mongodb.php
diff --git a/tests/Functional/JsonApiTest.php b/tests/Functional/JsonApiTest.php
index 9b97c347a28..227a6b8e232 100644
--- a/tests/Functional/JsonApiTest.php
+++ b/tests/Functional/JsonApiTest.php
@@ -14,7 +14,10 @@
 namespace ApiPlatform\Tests\Functional;
 
 use ApiPlatform\Symfony\Bundle\Test\ApiTestCase;
+use ApiPlatform\Tests\Fixtures\TestBundle\ApiResource\JsonApiDummy;
 use ApiPlatform\Tests\Fixtures\TestBundle\ApiResource\JsonApiErrorTestResource;
+use ApiPlatform\Tests\Fixtures\TestBundle\ApiResource\JsonApiNotExposedRelation;
+use ApiPlatform\Tests\Fixtures\TestBundle\ApiResource\JsonApiRelatedDummy;
 use ApiPlatform\Tests\SetupClassResourcesTrait;
 
 class JsonApiTest extends ApiTestCase
@@ -29,6 +32,9 @@ public static function getResources(): array
     {
         return [
             JsonApiErrorTestResource::class,
+            JsonApiDummy::class,
+            JsonApiRelatedDummy::class,
+            JsonApiNotExposedRelation::class,
         ];
     }
 
@@ -50,4 +56,134 @@ public function testError(): void
             ],
         ]);
     }
+
+    public function testGetSingleResourceIdentifierMode(): void
+    {
+        $this->bootJsonApiKernel();
+        self::createClient()->request('GET', '/jsonapi_dummies/10', [
+            'headers' => ['accept' => 'application/vnd.api+json'],
+        ]);
+
+        $this->assertResponseIsSuccessful();
+        $this->assertResponseHeaderSame('content-type', 'application/vnd.api+json; charset=utf-8');
+        $this->assertJsonContains([
+            'data' => [
+                'id' => '10',
+                'type' => 'JsonApiDummy',
+                'links' => [
+                    'self' => '/jsonapi_dummies/10',
+                ],
+                'attributes' => [
+                    'name' => 'Dummy #10',
+                ],
+            ],
+        ]);
+    }
+
+    public function testGetCollectionIdentifierMode(): void
+    {
+        $this->bootJsonApiKernel();
+        self::createClient()->request('GET', '/jsonapi_dummies', [
+            'headers' => ['accept' => 'application/vnd.api+json'],
+        ]);
+
+        $this->assertResponseIsSuccessful();
+        $this->assertResponseHeaderSame('content-type', 'application/vnd.api+json; charset=utf-8');
+        $this->assertJsonContains([
+            'data' => [
+                [
+                    'id' => '1',
+                    'type' => 'JsonApiDummy',
+                    'links' => [
+                        'self' => '/jsonapi_dummies/1',
+                    ],
+                ],
+                [
+                    'id' => '2',
+                    'type' => 'JsonApiDummy',
+                    'links' => [
+                        'self' => '/jsonapi_dummies/2',
+                    ],
+                ],
+            ],
+        ]);
+    }
+
+    public function testRelationWithNotExposedOperationIdentifierMode(): void
+    {
+        $this->bootJsonApiKernel();
+        self::createClient()->request('GET', '/jsonapi_dummies/10', [
+            'headers' => ['accept' => 'application/vnd.api+json'],
+        ]);
+
+        $this->assertResponseIsSuccessful();
+        $this->assertJsonContains([
+            'data' => [
+                'id' => '10',
+                'type' => 'JsonApiDummy',
+                'relationships' => [
+                    'notExposedRelation' => [
+                        'data' => [
+                            'id' => '5',
+                            'type' => 'JsonApiNotExposedRelation',
+                        ],
+                    ],
+                ],
+            ],
+        ]);
+    }
+
+    public function testSubresourceNotExposedIdentifierMode(): void
+    {
+        $this->bootJsonApiKernel();
+        self::createClient()->request('GET', '/jsonapi_dummies/10/not_exposed_relation', [
+            'headers' => ['accept' => 'application/vnd.api+json'],
+        ]);
+
+        $this->assertResponseIsSuccessful();
+        $this->assertJsonContains([
+            'data' => [
+                'id' => '5',
+                'type' => 'JsonApiNotExposedRelation',
+                // links.self uses the subresource URI — the only publicly accessible route
+                'links' => ['self' => '/jsonapi_dummies/10/not_exposed_relation'],
+            ],
+        ]);
+    }
+
+    public function testGetSingleResourceDefaultIriMode(): void
+    {
+        // Default mode (use_iri_as_id: true) — id should be the IRI, no links.self
+        self::createClient()->request('GET', '/jsonapi_dummies/10', [
+            'headers' => ['accept' => 'application/vnd.api+json'],
+        ]);
+
+        $this->assertResponseIsSuccessful();
+        $this->assertJsonContains([
+            'data' => [
+                'id' => '/jsonapi_dummies/10',
+                'type' => 'JsonApiDummy',
+            ],
+        ]);
+
+        // Verify no links.self is present on the data object
+        $json = json_decode(self::getClient()->getResponse()->getContent(), true);
+        $this->assertArrayNotHasKey('links', $json['data']);
+    }
+
+    private function bootJsonApiKernel(): void
+    {
+        $baseEnv = $_SERVER['APP_ENV'] ?? 'test';
+        $jsonApiEnv = 'mongodb' === $baseEnv ? 'jsonapi_mongodb' : 'jsonapi';
+
+        // AppKernel overrides environment with $_SERVER['APP_ENV'] (behat compat),
+        // so we must temporarily set it to our target environment.
+        $_SERVER['APP_ENV'] = $jsonApiEnv;
+
+        try {
+            self::bootKernel(['environment' => $jsonApiEnv]);
+        } finally {
+            $_SERVER['APP_ENV'] = $baseEnv;
+        }
+    }
 }
diff --git a/tests/Symfony/Bundle/DependencyInjection/ConfigurationTest.php b/tests/Symfony/Bundle/DependencyInjection/ConfigurationTest.php
index 10f3a129406..52b3d33f3fa 100644
--- a/tests/Symfony/Bundle/DependencyInjection/ConfigurationTest.php
+++ b/tests/Symfony/Bundle/DependencyInjection/ConfigurationTest.php
@@ -246,6 +246,9 @@ private function runDefaultConfigTests(array $doctrineIntegrationsToLoad = ['orm
             'mcp' => [
                 'enabled' => true,
             ],
+            'jsonapi' => [
+                'use_iri_as_id' => true,
+            ],
         ], $config);
     }