diff --git a/src/database/migrations/2026_05_22_000001_add_index_to_site_state_parties_world_heritage_site_id.php b/src/database/migrations/2026_05_22_000001_add_index_to_site_state_parties_world_heritage_site_id.php
new file mode 100644
index 0000000..13c8bc1
--- /dev/null
+++ b/src/database/migrations/2026_05_22_000001_add_index_to_site_state_parties_world_heritage_site_id.php
@@ -0,0 +1,22 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('site_state_parties', function (Blueprint $table) {
+            $table->index('world_heritage_site_id', 'idx_ssp_world_heritage_site_id');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('site_state_parties', function (Blueprint $table) {
+            $table->dropIndex('idx_ssp_world_heritage_site_id');
+        });
+    }
+};