diff --git a/src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php b/src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php
index 1a135ba546779..848b4e56d75a1 100644
--- a/src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php
+++ b/src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php
@@ -70,6 +70,8 @@ public function register_routes() {
 			$valid_image_sizes[] = 'original';
 			// Used for PDF thumbnails.
 			$valid_image_sizes[] = 'full';
+			// Client-side big image threshold: sideload the scaled version.
+			$valid_image_sizes[] = 'scaled';
 
 			register_rest_route(
 				$this->namespace,
@@ -2053,6 +2055,48 @@ public function sideload_item( WP_REST_Request $request ) {
 
 		if ( 'original' === $image_size ) {
 			$metadata['original_image'] = wp_basename( $path );
+		} elseif ( 'scaled' === $image_size ) {
+			// The current attached file is the original; record it as original_image.
+			$current_file = get_attached_file( $attachment_id, true );
+
+			if ( ! $current_file ) {
+				return new WP_Error(
+					'rest_sideload_no_attached_file',
+					__( 'Unable to retrieve the attached file for this attachment.' ),
+					array( 'status' => 404 )
+				);
+			}
+
+			$metadata['original_image'] = wp_basename( $current_file );
+
+			// Validate the scaled image before updating the attached file.
+			$size     = wp_getimagesize( $path );
+			$filesize = wp_filesize( $path );
+
+			if ( ! $size || ! $filesize ) {
+				return new WP_Error(
+					'rest_sideload_invalid_image',
+					__( 'Unable to read the scaled image file.' ),
+					array( 'status' => 500 )
+				);
+			}
+
+			// Update the attached file to point to the scaled version.
+			if (
+				get_attached_file( $attachment_id, true ) !== $path &&
+				! update_attached_file( $attachment_id, $path )
+			) {
+				return new WP_Error(
+					'rest_sideload_update_attached_file_failed',
+					__( 'Unable to update the attached file for this attachment.' ),
+					array( 'status' => 500 )
+				);
+			}
+
+			$metadata['width']    = $size[0];
+			$metadata['height']   = $size[1];
+			$metadata['filesize'] = $filesize;
+			$metadata['file']     = _wp_relative_upload_path( $path );
 		} else {
 			$metadata['sizes'] = $metadata['sizes'] ?? array();
 
@@ -2110,7 +2154,7 @@ public function sideload_item( WP_REST_Request $request ) {
 	 * @return string Filtered file name.
 	 */
 	private static function filter_wp_unique_filename( $filename, $dir, $number, $attachment_filename ) {
-		if ( empty( $number ) || ! $attachment_filename ) {
+		if ( ! is_int( $number ) || ! $attachment_filename ) {
 			return $filename;
 		}
 
@@ -2123,8 +2167,8 @@ private static function filter_wp_unique_filename( $filename, $dir, $number, $at
 		}
 
 		$matches = array();
-		if ( preg_match( '/(.*)(-\d+x\d+)-' . $number . '$/', $name, $matches ) ) {
-			$filename_without_suffix = $matches[1] . $matches[2] . ".$ext";
+		if ( preg_match( '/(.*)-(\d+x\d+|scaled)-' . $number . '$/', $name, $matches ) ) {
+			$filename_without_suffix = $matches[1] . '-' . $matches[2] . ".$ext";
 			if ( $matches[1] === $orig_name && ! file_exists( "$dir/$filename_without_suffix" ) ) {
 				return $filename_without_suffix;
 			}
diff --git a/tests/phpunit/tests/rest-api/rest-attachments-controller.php b/tests/phpunit/tests/rest-api/rest-attachments-controller.php
index 796e58c45d97d..93cd4211c93ba 100644
--- a/tests/phpunit/tests/rest-api/rest-attachments-controller.php
+++ b/tests/phpunit/tests/rest-api/rest-attachments-controller.php
@@ -3154,4 +3154,193 @@ static function ( $data ) use ( &$captured_data ) {
 		// Verify that the data is an array (not an object).
 		$this->assertIsArray( $captured_data, 'Data passed to wp_insert_attachment should be an array' );
 	}
+
+	/**
+	 * Tests sideloading a scaled image for an existing attachment.
+	 *
+	 * @ticket 64737
+	 * @requires function imagejpeg
+	 */
+	public function test_sideload_scaled_image() {
+		wp_set_current_user( self::$author_id );
+
+		// First, create an attachment.
+		$request = new WP_REST_Request( 'POST', '/wp/v2/media' );
+		$request->set_header( 'Content-Type', 'image/jpeg' );
+		$request->set_header( 'Content-Disposition', 'attachment; filename=canola.jpg' );
+		$request->set_body( file_get_contents( self::$test_file ) );
+		$response      = rest_get_server()->dispatch( $request );
+		$data          = $response->get_data();
+		$attachment_id = $data['id'];
+
+		$this->assertSame( 201, $response->get_status() );
+
+		$original_file = get_attached_file( $attachment_id, true );
+
+		// Sideload a "scaled" version of the image.
+		$request = new WP_REST_Request( 'POST', "/wp/v2/media/{$attachment_id}/sideload" );
+		$request->set_header( 'Content-Type', 'image/jpeg' );
+		$request->set_header( 'Content-Disposition', 'attachment; filename=canola-scaled.jpg' );
+		$request->set_param( 'image_size', 'scaled' );
+		$request->set_body( file_get_contents( self::$test_file ) );
+		$response = rest_get_server()->dispatch( $request );
+
+		$this->assertSame( 200, $response->get_status(), 'Sideloading scaled image should succeed.' );
+
+		$metadata = wp_get_attachment_metadata( $attachment_id );
+
+		// The original file should now be recorded as original_image.
+		$this->assertArrayHasKey( 'original_image', $metadata, 'Metadata should contain original_image.' );
+		$this->assertSame( wp_basename( $original_file ), $metadata['original_image'], 'original_image should be the basename of the original attached file.' );
+
+		// The attached file should now point to the scaled version.
+		$new_file = get_attached_file( $attachment_id, true );
+		$this->assertStringContainsString( 'scaled', wp_basename( $new_file ), 'Attached file should now be the scaled version.' );
+
+		// Metadata should have width, height, filesize, and file updated.
+		$this->assertArrayHasKey( 'width', $metadata, 'Metadata should contain width.' );
+		$this->assertArrayHasKey( 'height', $metadata, 'Metadata should contain height.' );
+		$this->assertArrayHasKey( 'filesize', $metadata, 'Metadata should contain filesize.' );
+		$this->assertArrayHasKey( 'file', $metadata, 'Metadata should contain file.' );
+		$this->assertStringContainsString( 'scaled', $metadata['file'], 'Metadata file should reference the scaled version.' );
+		$this->assertGreaterThan( 0, $metadata['width'], 'Width should be positive.' );
+		$this->assertGreaterThan( 0, $metadata['height'], 'Height should be positive.' );
+		$this->assertGreaterThan( 0, $metadata['filesize'], 'Filesize should be positive.' );
+	}
+
+	/**
+	 * Tests that sideloading scaled image requires authentication.
+	 *
+	 * @ticket 64737
+	 * @requires function imagejpeg
+	 */
+	public function test_sideload_scaled_image_requires_auth() {
+		wp_set_current_user( self::$author_id );
+
+		// Create an attachment.
+		$request = new WP_REST_Request( 'POST', '/wp/v2/media' );
+		$request->set_header( 'Content-Type', 'image/jpeg' );
+		$request->set_header( 'Content-Disposition', 'attachment; filename=canola.jpg' );
+		$request->set_body( file_get_contents( self::$test_file ) );
+		$response      = rest_get_server()->dispatch( $request );
+		$attachment_id = $response->get_data()['id'];
+
+		// Try sideloading without authentication.
+		wp_set_current_user( 0 );
+
+		$request = new WP_REST_Request( 'POST', "/wp/v2/media/{$attachment_id}/sideload" );
+		$request->set_header( 'Content-Type', 'image/jpeg' );
+		$request->set_header( 'Content-Disposition', 'attachment; filename=canola-scaled.jpg' );
+		$request->set_param( 'image_size', 'scaled' );
+		$request->set_body( file_get_contents( self::$test_file ) );
+		$response = rest_get_server()->dispatch( $request );
+
+		$this->assertErrorResponse( 'rest_cannot_edit_image', $response, 401 );
+	}
+
+	/**
+	 * Tests that the sideload endpoint includes 'scaled' in the image_size enum.
+	 *
+	 * @ticket 64737
+	 */
+	public function test_sideload_route_includes_scaled_enum() {
+		$server = rest_get_server();
+		$routes = $server->get_routes();
+
+		$endpoint = '/wp/v2/media/(?P<id>[\d]+)/sideload';
+		$this->assertArrayHasKey( $endpoint, $routes, 'Sideload route should exist.' );
+
+		$route    = $routes[ $endpoint ];
+		$endpoint = $route[0];
+		$args     = $endpoint['args'];
+
+		$param_name = 'image_size';
+		$this->assertArrayHasKey( $param_name, $args, 'Route should have image_size arg.' );
+		$this->assertContains( 'scaled', $args[ $param_name ]['enum'], 'image_size enum should include scaled.' );
+	}
+
+	/**
+	 * Tests the filter_wp_unique_filename method handles the -scaled suffix.
+	 *
+	 * @ticket 64737
+	 * @requires function imagejpeg
+	 */
+	public function test_sideload_scaled_unique_filename() {
+		wp_set_current_user( self::$author_id );
+
+		// Create an attachment.
+		$request = new WP_REST_Request( 'POST', '/wp/v2/media' );
+		$request->set_header( 'Content-Type', 'image/jpeg' );
+		$request->set_header( 'Content-Disposition', 'attachment; filename=canola.jpg' );
+		$request->set_body( file_get_contents( self::$test_file ) );
+		$response      = rest_get_server()->dispatch( $request );
+		$attachment_id = $response->get_data()['id'];
+
+		// Sideload with the -scaled suffix.
+		$request = new WP_REST_Request( 'POST', "/wp/v2/media/{$attachment_id}/sideload" );
+		$request->set_header( 'Content-Type', 'image/jpeg' );
+		$request->set_header( 'Content-Disposition', 'attachment; filename=canola-scaled.jpg' );
+		$request->set_param( 'image_size', 'scaled' );
+		$request->set_body( file_get_contents( self::$test_file ) );
+		$response = rest_get_server()->dispatch( $request );
+
+		$this->assertSame( 200, $response->get_status(), 'Sideloading scaled image should succeed.' );
+
+		// The filename should retain the -scaled suffix without numeric disambiguation.
+		$new_file = get_attached_file( $attachment_id, true );
+		$basename = wp_basename( $new_file );
+		$this->assertMatchesRegularExpression( '/canola-scaled\.jpg$/', $basename, 'Scaled filename should not have numeric suffix appended.' );
+	}
+
+	/**
+	 * Tests that sideloading a scaled image for a different attachment retains the numeric suffix
+	 * when a file with the same name already exists on disk.
+	 *
+	 * @ticket 64737
+	 * @requires function imagejpeg
+	 */
+	public function test_sideload_scaled_unique_filename_conflict() {
+		wp_set_current_user( self::$author_id );
+
+		// Create the first attachment.
+		$request = new WP_REST_Request( 'POST', '/wp/v2/media' );
+		$request->set_header( 'Content-Type', 'image/jpeg' );
+		$request->set_header( 'Content-Disposition', 'attachment; filename=canola.jpg' );
+		$request->set_body( file_get_contents( self::$test_file ) );
+		$response        = rest_get_server()->dispatch( $request );
+		$attachment_id_a = $response->get_data()['id'];
+
+		// Sideload a scaled image for attachment A, creating canola-scaled.jpg on disk.
+		$request = new WP_REST_Request( 'POST', "/wp/v2/media/{$attachment_id_a}/sideload" );
+		$request->set_header( 'Content-Type', 'image/jpeg' );
+		$request->set_header( 'Content-Disposition', 'attachment; filename=canola-scaled.jpg' );
+		$request->set_param( 'image_size', 'scaled' );
+		$request->set_body( file_get_contents( self::$test_file ) );
+		$response = rest_get_server()->dispatch( $request );
+
+		$this->assertSame( 200, $response->get_status(), 'First sideload should succeed.' );
+
+		// Create a second, different attachment.
+		$request = new WP_REST_Request( 'POST', '/wp/v2/media' );
+		$request->set_header( 'Content-Type', 'image/jpeg' );
+		$request->set_header( 'Content-Disposition', 'attachment; filename=other.jpg' );
+		$request->set_body( file_get_contents( self::$test_file ) );
+		$response        = rest_get_server()->dispatch( $request );
+		$attachment_id_b = $response->get_data()['id'];
+
+		// Sideload scaled for attachment B using the same filename that already exists on disk.
+		$request = new WP_REST_Request( 'POST', "/wp/v2/media/{$attachment_id_b}/sideload" );
+		$request->set_header( 'Content-Type', 'image/jpeg' );
+		$request->set_header( 'Content-Disposition', 'attachment; filename=canola-scaled.jpg' );
+		$request->set_param( 'image_size', 'scaled' );
+		$request->set_body( file_get_contents( self::$test_file ) );
+		$response = rest_get_server()->dispatch( $request );
+
+		$this->assertSame( 200, $response->get_status(), 'Second sideload should succeed.' );
+
+		// The filename should have a numeric suffix since the base name does not match this attachment.
+		$new_file = get_attached_file( $attachment_id_b, true );
+		$basename = wp_basename( $new_file );
+		$this->assertMatchesRegularExpression( '/canola-scaled-\d+\.jpg$/', $basename, 'Scaled filename should have numeric suffix when file conflicts with a different attachment.' );
+	}
 }
diff --git a/tests/qunit/fixtures/wp-api-generated.js b/tests/qunit/fixtures/wp-api-generated.js
index ef59ce5ca5074..40e20d10d4dc4 100644
--- a/tests/qunit/fixtures/wp-api-generated.js
+++ b/tests/qunit/fixtures/wp-api-generated.js
@@ -3703,7 +3703,8 @@ mockedApiResponse.Schema = {
                                 "1536x1536",
                                 "2048x2048",
                                 "original",
-                                "full"
+                                "full",
+                                "scaled"
                             ],
                             "required": true
                         },