diff --git a/tests/types/resolving/test_union_get_type_resolver.py b/tests/types/resolving/test_union_get_type_resolver.py
new file mode 100644
index 0000000000..7cae5cd33a
--- /dev/null
+++ b/tests/types/resolving/test_union_get_type_resolver.py
@@ -0,0 +1,372 @@
+"""Tests for StrawberryUnion.get_type_resolver method and the resolver function it returns."""
+
+from dataclasses import dataclass
+from typing import Annotated, Generic, TypeVar
+from unittest.mock import MagicMock
+
+import pytest
+
+import strawberry
+from strawberry.annotation import StrawberryAnnotation
+from strawberry.exceptions import UnallowedReturnTypeForUnion, WrongReturnTypeForUnion
+from strawberry.schema.types.concrete_type import TypeMap
+from strawberry.types.union import StrawberryUnion
+
+
+def test_get_type_resolver_returns_function():
+    """Test that get_type_resolver returns a callable function."""
+
+    @strawberry.type
+    class A:
+        a: int
+
+    @strawberry.type
+    class B:
+        b: int
+
+    union = StrawberryUnion(
+        type_annotations=(
+            StrawberryAnnotation(A),
+            StrawberryAnnotation(B),
+        )
+    )
+
+    type_map: TypeMap = {}
+    resolver = union.get_type_resolver(type_map)
+
+    assert callable(resolver)
+
+
+def test_type_resolver_with_object_definition():
+    """Test that the resolver correctly resolves types when root has object definition."""
+
+    @strawberry.type
+    class A:
+        a: int
+
+    @strawberry.type
+    class B:
+        b: int
+
+    # Create a schema to get proper GraphQL types
+    @strawberry.type
+    class Query:
+        @strawberry.field
+        def test(self) -> A | B:
+            return A(a=1)
+
+    schema = strawberry.Schema(query=Query)
+
+    # Find the union in the schema's type map
+    union_name = None
+    union_def = None
+    for name, concrete_type in schema.schema_converter.type_map.items():
+        if isinstance(concrete_type.definition, StrawberryUnion):
+            union_name = name
+            union_def = concrete_type.definition
+            break
+
+    assert union_name is not None, "Union should be in type map"
+    assert union_def is not None, "Union definition should be found"
+    graphql_union = schema.schema_converter.type_map[union_name].implementation
+
+    # Get the resolver from the union
+    resolver = union_def.get_type_resolver(schema.schema_converter.type_map)
+
+    # Create mock info
+    info = MagicMock()
+    info.field_name = "test"
+
+    # Test with A instance
+    a_instance = A(a=1)
+    result = resolver(a_instance, info, graphql_union)
+    assert result == "A"
+
+    # Test with B instance
+    b_instance = B(b=2)
+    result = resolver(b_instance, info, graphql_union)
+    assert result == "B"
+
+
+def test_type_resolver_with_is_type_of():
+    """Test that the resolver uses is_type_of when root doesn't have object definition."""
+
+    @dataclass
+    class ADataclass:
+        a: int
+
+    @dataclass
+    class BDataclass:
+        b: int
+
+    @strawberry.type
+    class A:
+        a: int
+
+        @classmethod
+        def is_type_of(cls, obj, _info) -> bool:
+            return isinstance(obj, ADataclass)
+
+    @strawberry.type
+    class B:
+        b: int
+
+        @classmethod
+        def is_type_of(cls, obj, _info) -> bool:
+            return isinstance(obj, BDataclass)
+
+    # Create a schema to get proper GraphQL types
+    @strawberry.type
+    class Query:
+        @strawberry.field
+        def test(self) -> A | B:
+            return ADataclass(a=1)  # type: ignore
+
+    schema = strawberry.Schema(query=Query)
+
+    # Find the union in the schema's type map
+    union_name = None
+    union_def = None
+    for name, concrete_type in schema.schema_converter.type_map.items():
+        if isinstance(concrete_type.definition, StrawberryUnion):
+            union_name = name
+            union_def = concrete_type.definition
+            break
+
+    assert union_name is not None, "Union should be in type map"
+    assert union_def is not None, "Union definition should be found"
+    graphql_union = schema.schema_converter.type_map[union_name].implementation
+
+    # Get the resolver from the union
+    resolver = union_def.get_type_resolver(schema.schema_converter.type_map)
+
+    # Create mock info
+    info = MagicMock()
+    info.field_name = "test"
+
+    # Test with ADataclass instance (should resolve to A)
+    a_dataclass = ADataclass(a=1)
+    result = resolver(a_dataclass, info, graphql_union)
+    assert result == "A"
+
+    # Test with BDataclass instance (should resolve to B)
+    b_dataclass = BDataclass(b=2)
+    result = resolver(b_dataclass, info, graphql_union)
+    assert result == "B"
+
+
+def test_type_resolver_raises_wrong_return_type_when_no_is_type_of_matches():
+    """Test that the resolver raises WrongReturnTypeForUnion when is_type_of doesn't match."""
+
+    @dataclass
+    class UnknownDataclass:
+        x: int
+
+    @strawberry.type
+    class A:
+        a: int
+
+        @classmethod
+        def is_type_of(cls, obj, _info) -> bool:
+            return False  # This won't match
+
+    @strawberry.type
+    class B:
+        b: int
+
+        @classmethod
+        def is_type_of(cls, obj, _info) -> bool:
+            return False  # This won't match either
+
+    # Create a schema to get proper GraphQL types
+    @strawberry.type
+    class Query:
+        @strawberry.field
+        def test(self) -> A | B:
+            return UnknownDataclass(x=1)  # type: ignore
+
+    schema = strawberry.Schema(query=Query)
+
+    # Find the union in the schema's type map
+    union_name = None
+    union_def = None
+    for name, concrete_type in schema.schema_converter.type_map.items():
+        if isinstance(concrete_type.definition, StrawberryUnion):
+            union_name = name
+            union_def = concrete_type.definition
+            break
+
+    assert union_name is not None, "Union should be in type map"
+    assert union_def is not None, "Union definition should be found"
+    graphql_union = schema.schema_converter.type_map[union_name].implementation
+
+    # Get the resolver from the union
+    resolver = union_def.get_type_resolver(schema.schema_converter.type_map)
+
+    # Create mock info
+    info = MagicMock()
+    info.field_name = "test"
+
+    # Test with UnknownDataclass instance (should raise error)
+    unknown_instance = UnknownDataclass(x=1)
+    with pytest.raises(WrongReturnTypeForUnion) as exc_info:
+        resolver(unknown_instance, info, graphql_union)
+
+    assert "test" in str(exc_info.value)
+    assert "UnknownDataclass" in str(exc_info.value)
+
+
+def test_type_resolver_raises_unallowed_return_type_when_not_in_union():
+    """Test that the resolver raises UnallowedReturnTypeForUnion when type is not in union."""
+
+    @strawberry.type
+    class A:
+        a: int
+
+    @strawberry.type
+    class B:
+        b: int
+
+    @strawberry.type
+    class Outside:
+        c: int
+
+    # Create a schema that includes Outside but not in the union
+    @strawberry.type
+    class Query:
+        @strawberry.field
+        def test(self) -> A | B:
+            return Outside(c=1)  # type: ignore
+
+    schema = strawberry.Schema(query=Query, types=[Outside])
+
+    # Find the union in the schema's type map
+    union_name = None
+    union_def = None
+    for name, concrete_type in schema.schema_converter.type_map.items():
+        if isinstance(concrete_type.definition, StrawberryUnion):
+            union_name = name
+            union_def = concrete_type.definition
+            break
+
+    assert union_name is not None, "Union should be in type map"
+    assert union_def is not None, "Union definition should be found"
+    graphql_union = schema.schema_converter.type_map[union_name].implementation
+
+    # Get the resolver from the union
+    resolver = union_def.get_type_resolver(schema.schema_converter.type_map)
+
+    # Create mock info
+    info = MagicMock()
+    info.field_name = "test"
+
+    # Test with Outside instance (should raise error)
+    outside_instance = Outside(c=1)
+    with pytest.raises(UnallowedReturnTypeForUnion) as exc_info:
+        resolver(outside_instance, info, graphql_union)
+
+    assert "test" in str(exc_info.value)
+    assert "Outside" in str(exc_info.value)
+    assert "A" in str(exc_info.value) or "B" in str(exc_info.value)
+
+
+def test_type_resolver_prioritizes_union_types():
+    """Test that the resolver prioritizes union types over other types in type_map.
+
+    This tests the bug fix from PR #1463 where union types should be checked first
+    before falling back to all types in the type_map.
+    """
+    T = TypeVar("T")
+
+    @strawberry.type
+    class Container(Generic[T]):
+        items: list[T]
+
+    @strawberry.type
+    class A:
+        a: str
+
+    @strawberry.type
+    class B:
+        b: int
+
+    @strawberry.type
+    class Query:
+        @strawberry.field
+        def container_a(self) -> Container[A] | A:
+            return Container(items=[A(a="hello")])
+
+        @strawberry.field
+        def container_b(self) -> Container[B] | B:
+            return Container(items=[B(b=3)])
+
+    schema = strawberry.Schema(query=Query)
+
+    # Find the union for Container[A] | A in the schema's type map
+    # We'll look for a union that has "AContainer" in its name (generated by schema)
+    union_name = None
+    union_def = None
+    for name, concrete_type in schema.schema_converter.type_map.items():
+        if isinstance(concrete_type.definition, StrawberryUnion):
+            # Check if this union name contains "AContainer" (indicating Container[A] | A)
+            # and check if A is in the union types
+            union_types = concrete_type.definition.types
+            has_a = A in union_types
+            if "AContainer" in name and has_a:
+                union_name = name
+                union_def = concrete_type.definition
+                break
+
+    assert union_name is not None, "Union Container[A] | A should be in type map"
+    assert union_def is not None, "Union definition should be found"
+    graphql_union = schema.schema_converter.type_map[union_name].implementation
+
+    # Get the resolver from the union
+    resolver = union_def.get_type_resolver(schema.schema_converter.type_map)
+
+    # Create mock info
+    info = MagicMock()
+    info.field_name = "containerA"
+
+    # Test with Container[A] instance - should resolve to Container[A], not Container[B]
+    container_a_instance = Container(items=[A(a="hello")])
+    result = resolver(container_a_instance, info, graphql_union)
+
+    # Should resolve to the Container[A] type name
+    assert result == "AContainer"
+
+    # Test with A instance
+    a_instance = A(a="hello")
+    result = resolver(a_instance, info, graphql_union)
+    assert result == "A"
+
+
+def test_type_resolver_with_single_type_union():
+    """Test that the resolver works correctly with a single-type union."""
+
+    @strawberry.type
+    class A:
+        a: int
+
+    @strawberry.type
+    class Query:
+        @strawberry.field
+        def test(self) -> Annotated[A, strawberry.union(name="SingleUnion")]:
+            return A(a=1)
+
+    schema = strawberry.Schema(query=Query)
+    graphql_union = schema.schema_converter.type_map["SingleUnion"].implementation
+    union_def = schema.schema_converter.type_map["SingleUnion"].definition
+    assert isinstance(union_def, StrawberryUnion), "Should be a StrawberryUnion"
+
+    # Get the resolver from the union
+    resolver = union_def.get_type_resolver(schema.schema_converter.type_map)
+
+    # Create mock info
+    info = MagicMock()
+    info.field_name = "test"
+
+    # Test with A instance
+    a_instance = A(a=1)
+    result = resolver(a_instance, info, graphql_union)
+    assert result == "A"