diff --git a/src/JsonApi/JsonSchema/SchemaFactory.php b/src/JsonApi/JsonSchema/SchemaFactory.php
index eccfb67d3db..e86aedfe6d7 100644
--- a/src/JsonApi/JsonSchema/SchemaFactory.php
+++ b/src/JsonApi/JsonSchema/SchemaFactory.php
@@ -63,6 +63,7 @@ final class SchemaFactory implements SchemaFactoryInterface, SchemaFactoryAwareI
                 'format' => 'iri-reference',
             ],
         ],
+        'required' => ['type', 'id'],
     ];
     private const PROPERTY_PROPS = [
         'id' => [
@@ -321,7 +322,10 @@ private function buildDefinitionPropertiesSchema(string $key, string $className,
 
                     $refs[$this->getSchemaUriPrefix($schema->getVersion()).$definitionName] = '$ref';
                 }
-                $relatedDefinitions[$propertyName] = array_flip($refs);
+                // keep one entry per related definition: a polymorphic relation targets several resource classes, all of which may appear in "included"
+                foreach (array_keys($refs) as $ref) {
+                    $relatedDefinitions[$ref] = ['$ref' => $ref];
+                }
                 if ($isOne) {
                     $relationships[$propertyName]['properties']['data'] = [
                         'oneOf' => [
diff --git a/src/JsonApi/Tests/Fixtures/OtherRelatedDummy.php b/src/JsonApi/Tests/Fixtures/OtherRelatedDummy.php
new file mode 100644
index 00000000000..aad2f3e4d52
--- /dev/null
+++ b/src/JsonApi/Tests/Fixtures/OtherRelatedDummy.php
@@ -0,0 +1,37 @@
+<?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\JsonApi\Tests\Fixtures;
+
+use ApiPlatform\Metadata\ApiResource;
+
+/**
+ * A second related resource used to exercise polymorphic (union-typed) relationships.
+ */
+#[ApiResource]
+class OtherRelatedDummy
+{
+    private ?int $id = null;
+
+    public ?string $label = null;
+
+    public function getId(): ?int
+    {
+        return $this->id;
+    }
+
+    public function setId(?int $id): void
+    {
+        $this->id = $id;
+    }
+}
diff --git a/src/JsonApi/Tests/JsonSchema/SchemaFactoryTest.php b/src/JsonApi/Tests/JsonSchema/SchemaFactoryTest.php
index 5c647242691..648cfd87701 100644
--- a/src/JsonApi/Tests/JsonSchema/SchemaFactoryTest.php
+++ b/src/JsonApi/Tests/JsonSchema/SchemaFactoryTest.php
@@ -15,6 +15,7 @@
 
 use ApiPlatform\JsonApi\JsonSchema\SchemaFactory;
 use ApiPlatform\JsonApi\Tests\Fixtures\Dummy;
+use ApiPlatform\JsonApi\Tests\Fixtures\OtherRelatedDummy;
 use ApiPlatform\JsonApi\Tests\Fixtures\RelatedDummy;
 use ApiPlatform\JsonSchema\DefinitionNameFactory;
 use ApiPlatform\JsonSchema\Schema;
@@ -230,6 +231,110 @@ public function testRelationIsExcludedFromAttributes(): void
         $this->assertArrayHasKey('relatedDummy', $dataProperties['relationships']['properties']);
     }
 
+    public function testRelationshipLinkageRequiresTypeAndId(): void
+    {
+        $schemaFactory = $this->buildSchemaFactoryWithRelation();
+        $resultSchema = $schemaFactory->buildSchema(Dummy::class, 'jsonapi');
+
+        $definitions = $resultSchema->getDefinitions();
+        $rootDefinitionKey = $resultSchema->getRootDefinitionKey();
+        $dataProperties = $definitions[$rootDefinitionKey]['properties']['data']['properties'];
+
+        // a resource identifier object MUST contain type and id, @see https://jsonapi.org/format/#document-resource-identifier-objects
+        $linkage = $dataProperties['relationships']['properties']['relatedDummy']['properties']['data']['oneOf'][1];
+        $this->assertSame('object', $linkage['type']);
+        $this->assertSame(['type', 'id'], $linkage['required']);
+    }
+
+    public function testIncludedListsAllPolymorphicRelationTargets(): void
+    {
+        $schemaFactory = $this->buildSchemaFactoryWithPolymorphicRelation();
+        $resultSchema = $schemaFactory->buildSchema(Dummy::class, 'jsonapi');
+
+        $definitions = $resultSchema->getDefinitions();
+        $rootDefinitionKey = $resultSchema->getRootDefinitionKey();
+        $included = $definitions[$rootDefinitionKey]['properties']['included'];
+
+        $refs = array_column($included['items']['anyOf'], '$ref');
+        $this->assertContains('#/definitions/RelatedDummy.jsonapi', $refs);
+        $this->assertContains('#/definitions/OtherRelatedDummy.jsonapi', $refs, 'every target of a polymorphic relation must be listed in included');
+    }
+
+    private function buildSchemaFactoryWithPolymorphicRelation(): SchemaFactory
+    {
+        $dummyOperation = (new Get())->withName('get')->withShortName('Dummy');
+        $relatedOperation = (new Get())->withName('get')->withShortName('RelatedDummy');
+        $otherRelatedOperation = (new Get())->withName('get')->withShortName('OtherRelatedDummy');
+
+        $resourceMetadataFactory = $this->prophesize(ResourceMetadataCollectionFactoryInterface::class);
+        $resourceMetadataFactory->create(Dummy::class)->willReturn(
+            new ResourceMetadataCollection(Dummy::class, [
+                (new ApiResource())->withOperations(new Operations(['get' => $dummyOperation])),
+            ])
+        );
+        $resourceMetadataFactory->create(RelatedDummy::class)->willReturn(
+            new ResourceMetadataCollection(RelatedDummy::class, [
+                (new ApiResource())->withOperations(new Operations(['get' => $relatedOperation])),
+            ])
+        );
+        $resourceMetadataFactory->create(OtherRelatedDummy::class)->willReturn(
+            new ResourceMetadataCollection(OtherRelatedDummy::class, [
+                (new ApiResource())->withOperations(new Operations(['get' => $otherRelatedOperation])),
+            ])
+        );
+
+        $propertyNameCollectionFactory = $this->prophesize(PropertyNameCollectionFactoryInterface::class);
+        $propertyNameCollectionFactory->create(Dummy::class, Argument::type('array'))->willReturn(new PropertyNameCollection(['id', 'name', 'relatedDummy']));
+        $propertyNameCollectionFactory->create(RelatedDummy::class, Argument::type('array'))->willReturn(new PropertyNameCollection(['id', 'name']));
+        $propertyNameCollectionFactory->create(OtherRelatedDummy::class, Argument::type('array'))->willReturn(new PropertyNameCollection(['id', 'label']));
+
+        $propertyMetadataFactory = $this->prophesize(PropertyMetadataFactoryInterface::class);
+        $propertyMetadataFactory->create(Dummy::class, 'id', Argument::type('array'))->willReturn(
+            (new ApiProperty())->withNativeType(Type::int())->withReadable(true)->withSchema(['type' => 'integer'])
+        );
+        $propertyMetadataFactory->create(Dummy::class, 'name', Argument::type('array'))->willReturn(
+            (new ApiProperty())->withNativeType(Type::string())->withReadable(true)->withSchema(['type' => 'string'])
+        );
+        $propertyMetadataFactory->create(Dummy::class, 'relatedDummy', Argument::type('array'))->willReturn(
+            (new ApiProperty())->withNativeType(Type::union(Type::object(RelatedDummy::class), Type::object(OtherRelatedDummy::class)))->withReadable(true)->withSchema(['type' => Schema::UNKNOWN_TYPE])
+        );
+        $propertyMetadataFactory->create(RelatedDummy::class, 'id', Argument::type('array'))->willReturn(
+            (new ApiProperty())->withNativeType(Type::int())->withReadable(true)->withSchema(['type' => 'integer'])
+        );
+        $propertyMetadataFactory->create(RelatedDummy::class, 'name', Argument::type('array'))->willReturn(
+            (new ApiProperty())->withNativeType(Type::string())->withReadable(true)->withSchema(['type' => 'string'])
+        );
+        $propertyMetadataFactory->create(OtherRelatedDummy::class, 'id', Argument::type('array'))->willReturn(
+            (new ApiProperty())->withNativeType(Type::int())->withReadable(true)->withSchema(['type' => 'integer'])
+        );
+        $propertyMetadataFactory->create(OtherRelatedDummy::class, 'label', Argument::type('array'))->willReturn(
+            (new ApiProperty())->withNativeType(Type::string())->withReadable(true)->withSchema(['type' => 'string'])
+        );
+
+        $resourceClassResolver = $this->prophesize(ResourceClassResolverInterface::class);
+        $resourceClassResolver->isResourceClass(Dummy::class)->willReturn(true);
+        $resourceClassResolver->isResourceClass(RelatedDummy::class)->willReturn(true);
+        $resourceClassResolver->isResourceClass(OtherRelatedDummy::class)->willReturn(true);
+
+        $definitionNameFactory = new DefinitionNameFactory(null);
+
+        $baseSchemaFactory = new BaseSchemaFactory(
+            resourceMetadataFactory: $resourceMetadataFactory->reveal(),
+            propertyNameCollectionFactory: $propertyNameCollectionFactory->reveal(),
+            propertyMetadataFactory: $propertyMetadataFactory->reveal(),
+            resourceClassResolver: $resourceClassResolver->reveal(),
+            definitionNameFactory: $definitionNameFactory,
+        );
+
+        return new SchemaFactory(
+            schemaFactory: $baseSchemaFactory,
+            propertyMetadataFactory: $propertyMetadataFactory->reveal(),
+            resourceClassResolver: $resourceClassResolver->reveal(),
+            resourceMetadataFactory: $resourceMetadataFactory->reveal(),
+            definitionNameFactory: $definitionNameFactory,
+        );
+    }
+
     private function buildSchemaFactoryWithRelation(): SchemaFactory
     {
         $dummyOperation = (new Get())->withName('get')->withShortName('Dummy');