diff --git a/src/Mappers/Parameters/TypeHandler.php b/src/Mappers/Parameters/TypeHandler.php
index 5a85b682fc..28e85f3b3a 100644
--- a/src/Mappers/Parameters/TypeHandler.php
+++ b/src/Mappers/Parameters/TypeHandler.php
@@ -27,6 +27,7 @@
 use ReflectionParameter;
 use ReflectionProperty;
 use ReflectionType;
+use ReflectionUnionType;
 use TheCodingMachine\GraphQLite\Annotations\HideParameter;
 use TheCodingMachine\GraphQLite\Annotations\ParameterAnnotations;
 use TheCodingMachine\GraphQLite\Annotations\UseInputType;
@@ -42,6 +43,7 @@
 use TheCodingMachine\GraphQLite\Types\TypeResolver;
 use Webmozart\Assert\Assert;
 
+use function array_map;
 use function array_merge;
 use function array_unique;
 use function assert;
@@ -408,17 +410,36 @@ private function appendTypes(Type $type, ?Type $docBlockType): Type
      */
     private function reflectionTypeToPhpDocType(ReflectionType $type, ReflectionClass $reflectionClass): Type
     {
-        assert($type instanceof ReflectionNamedType);
-        $phpdocType = $this->phpDocumentorTypeResolver->resolve($type->getName());
-        Assert::notNull($phpdocType);
+        assert($type instanceof ReflectionNamedType || $type instanceof ReflectionUnionType);
+        if ($type instanceof ReflectionNamedType) {
+            $phpdocType = $this->phpDocumentorTypeResolver->resolve($type->getName());
+            Assert::notNull($phpdocType);
 
-        $phpdocType = $this->resolveSelf($phpdocType, $reflectionClass);
+            $phpdocType = $this->resolveSelf($phpdocType, $reflectionClass);
 
-        if ($type->allowsNull()) {
-            $phpdocType = new Nullable($phpdocType);
-        }
+            if ($type->allowsNull()) {
+                $phpdocType = new Nullable($phpdocType);
+            }
 
-        return $phpdocType;
+            return $phpdocType;
+        }
+        return new Compound(
+            array_map(
+                function ($namedType) use ($reflectionClass): Type {
+                    \assert($namedType instanceof ReflectionNamedType);
+                    $phpdocType = $this->phpDocumentorTypeResolver->resolve($namedType->getName());
+                    Assert::notNull($phpdocType);
+
+                    $phpdocType = $this->resolveSelf($phpdocType, $reflectionClass);
+
+                    if ($namedType->allowsNull()) {
+                        $phpdocType = new Nullable($phpdocType);
+                    }
+                    return $phpdocType;
+                },
+                $type->getTypes()
+            )
+        );
     }
 
     /**
diff --git a/tests/Fixtures80/UnionOutputType.php b/tests/Fixtures80/UnionOutputType.php
new file mode 100644
index 0000000000..b3d4161a70
--- /dev/null
+++ b/tests/Fixtures80/UnionOutputType.php
@@ -0,0 +1,15 @@
+<?php
+
+declare(strict_types=1);
+
+namespace TheCodingMachine\GraphQLite\Fixtures80;
+
+use TheCodingMachine\GraphQLite\Fixtures\TestObject;
+use TheCodingMachine\GraphQLite\Fixtures\TestObject2;
+
+class UnionOutputType
+{
+    public function objectUnion(): TestObject|TestObject2 {
+        return new TestObject((''));
+    }
+}
diff --git a/tests/Mappers/Parameters/TypeMapperTest.php b/tests/Mappers/Parameters/TypeMapperTest.php
index 284cc5ef5f..5183c85319 100644
--- a/tests/Mappers/Parameters/TypeMapperTest.php
+++ b/tests/Mappers/Parameters/TypeMapperTest.php
@@ -3,13 +3,16 @@
 namespace TheCodingMachine\GraphQLite\Mappers\Parameters;
 
 use DateTimeImmutable;
+use GraphQL\Type\Definition\NonNull;
 use GraphQL\Type\Definition\ResolveInfo;
+use GraphQL\Type\Definition\UnionType;
 use ReflectionMethod;
 use Symfony\Component\Cache\Adapter\ArrayAdapter;
 use Symfony\Component\Cache\Psr16Cache;
 use Symfony\Component\Cache\Simple\ArrayCache;
 use TheCodingMachine\GraphQLite\AbstractQueryProviderTest;
 use TheCodingMachine\GraphQLite\Annotations\HideParameter;
+use TheCodingMachine\GraphQLite\Fixtures80\UnionOutputType;
 use TheCodingMachine\GraphQLite\Mappers\CannotMapTypeException;
 use TheCodingMachine\GraphQLite\Mappers\Root\BaseTypeMapper;
 use TheCodingMachine\GraphQLite\Mappers\Root\CompositeRootTypeMapper;
@@ -34,6 +37,29 @@ public function testMapScalarUnionException(): void
         $typeMapper->mapReturnType($refMethod, $docBlockObj);
     }
 
+    /**
+     * @requires PHP >= 8.0
+     */
+    public function testMapObjectUnionWorks(): void
+    {
+        $typeMapper = new TypeHandler($this->getArgumentResolver(), $this->getRootTypeMapper(), $this->getTypeResolver());
+
+        $cachedDocBlockFactory = new CachedDocBlockFactory(new Psr16Cache(new ArrayAdapter()));
+
+        $refMethod = new ReflectionMethod(UnionOutputType::class, 'objectUnion');
+        $docBlockObj = $cachedDocBlockFactory->getDocBlock($refMethod);
+
+        $gqType = $typeMapper->mapReturnType($refMethod, $docBlockObj);
+        $this->assertInstanceOf(NonNull::class, $gqType);
+        assert($gqType instanceof NonNull);
+        $memberType = $gqType->getOfType();
+        $this->assertInstanceOf(UnionType::class, $memberType);
+        assert($memberType instanceof UnionType);
+        $unionTypes = $memberType->getTypes();
+        $this->assertEquals('TestObject', $unionTypes[0]->name);
+        $this->assertEquals('TestObject2', $unionTypes[1]->name);
+    }
+
     public function testHideParameter(): void
     {
         $typeMapper = new TypeHandler($this->getArgumentResolver(), $this->getRootTypeMapper(), $this->getTypeResolver());