diff --git a/src/datamodel_code_generator/model/scalar.py b/src/datamodel_code_generator/model/scalar.py
index dcddddb4c..89a8ee7ae 100644
--- a/src/datamodel_code_generator/model/scalar.py
+++ b/src/datamodel_code_generator/model/scalar.py
@@ -67,9 +67,9 @@ def __init__(  # noqa: PLR0913
             extra_template_data[scalar_name] = defaultdict(dict)
 
         # py_type
-        py_type = extra_template_data[scalar_name].get(
+        py_type = extra_template_data[reference.original_name].get(
             "py_type",
-            DEFAULT_GRAPHQL_SCALAR_TYPES.get(reference.name, DEFAULT_GRAPHQL_SCALAR_TYPE),
+            DEFAULT_GRAPHQL_SCALAR_TYPES.get(reference.original_name, DEFAULT_GRAPHQL_SCALAR_TYPE),
         )
         extra_template_data[scalar_name]["py_type"] = py_type
 
diff --git a/src/datamodel_code_generator/parser/graphql.py b/src/datamodel_code_generator/parser/graphql.py
index b4e37abf8..d644ac232 100644
--- a/src/datamodel_code_generator/parser/graphql.py
+++ b/src/datamodel_code_generator/parser/graphql.py
@@ -479,8 +479,10 @@ def parse_input_object(self, input_graphql_object: graphql.GraphQLInputObjectTyp
 
     def parse_union(self, union_object: graphql.GraphQLUnionType) -> None:
         """Parse a GraphQL union type and add it to results."""
-        fields = [self.data_model_field_type(name=type_.name, data_type=DataType()) for type_ in union_object.types]
-
+        fields = [
+            self.data_model_field_type(name=self.references[type_.name].name, data_type=DataType())
+            for type_ in union_object.types
+        ]
         data_model_type = self.data_model_union_type(
             reference=self.references[union_object.name],
             fields=fields,
diff --git a/tests/data/expected/main/graphql/simple_star_wars_class_name_prefix.py b/tests/data/expected/main/graphql/simple_star_wars_class_name_prefix.py
new file mode 100644
index 000000000..891e908da
--- /dev/null
+++ b/tests/data/expected/main/graphql/simple_star_wars_class_name_prefix.py
@@ -0,0 +1,158 @@
+# generated by datamodel-codegen:
+#   filename:  simple-star-wars.graphql
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from typing import Literal, TypeAlias
+
+from pydantic import BaseModel, Field
+
+FooBoolean: TypeAlias = bool
+"""
+The `Boolean` scalar type represents `true` or `false`.
+"""
+
+
+FooID: TypeAlias = str
+"""
+The `ID` scalar type represents a unique identifier, often used to refetch an object or as key for a cache. The ID type appears in a JSON response as a String; however, it is not intended to be human-readable. When expected as an input type, any string (such as `"4"`) or integer (such as `4`) input value will be accepted as an ID.
+"""
+
+
+FooInt: TypeAlias = int
+"""
+The `Int` scalar type represents non-fractional signed whole numeric values. Int can represent values between -(2^31) and 2^31 - 1.
+"""
+
+
+FooString: TypeAlias = str
+"""
+The `String` scalar type represents textual data, represented as UTF-8 character sequences. The String type is most often used by GraphQL to represent free-form human-readable text.
+"""
+
+
+class FooFilm(BaseModel):
+    characters: list[FooPerson]
+    characters_ids: list[FooID]
+    director: FooString
+    episode_id: FooInt
+    id: FooID
+    opening_crawl: FooString
+    planets: list[FooPlanet]
+    planets_ids: list[FooID]
+    producer: FooString | None = None
+    release_date: FooString
+    species: list[FooSpecies]
+    species_ids: list[FooID]
+    starships: list[FooStarship]
+    starships_ids: list[FooID]
+    title: FooString
+    vehicles: list[FooVehicle]
+    vehicles_ids: list[FooID]
+    typename__: Literal['Film'] | None = Field('Film', alias='__typename')
+
+
+class FooPerson(BaseModel):
+    birth_year: FooString | None = None
+    eye_color: FooString | None = None
+    films: list[FooFilm]
+    films_ids: list[FooID]
+    gender: FooString | None = None
+    hair_color: FooString | None = None
+    height: FooInt | None = None
+    homeworld: FooPlanet | None = None
+    homeworld_id: FooID | None = None
+    id: FooID
+    mass: FooInt | None = None
+    name: FooString
+    skin_color: FooString | None = None
+    species: list[FooSpecies]
+    species_ids: list[FooID]
+    starships: list[FooStarship]
+    starships_ids: list[FooID]
+    vehicles: list[FooVehicle]
+    vehicles_ids: list[FooID]
+    typename__: Literal['Person'] | None = Field('Person', alias='__typename')
+
+
+class FooPlanet(BaseModel):
+    climate: FooString | None = None
+    diameter: FooString | None = None
+    films: list[FooFilm]
+    films_ids: list[FooID]
+    gravity: FooString | None = None
+    id: FooID
+    name: FooString
+    orbital_period: FooString | None = None
+    population: FooString | None = None
+    residents: list[FooPerson]
+    residents_ids: list[FooID]
+    rotation_period: FooString | None = None
+    surface_water: FooString | None = None
+    terrain: FooString | None = None
+    typename__: Literal['Planet'] | None = Field('Planet', alias='__typename')
+
+
+class FooSpecies(BaseModel):
+    average_height: FooString | None = None
+    average_lifespan: FooString | None = None
+    classification: FooString | None = None
+    designation: FooString | None = None
+    eye_colors: FooString | None = None
+    films: list[FooFilm]
+    films_ids: list[FooID]
+    hair_colors: FooString | None = None
+    id: FooID
+    language: FooString | None = None
+    name: FooString
+    people: list[FooPerson]
+    people_ids: list[FooID]
+    skin_colors: FooString | None = None
+    typename__: Literal['Species'] | None = Field('Species', alias='__typename')
+
+
+class FooStarship(BaseModel):
+    MGLT: FooString | None = None
+    cargo_capacity: FooString | None = None
+    consumables: FooString | None = None
+    cost_in_credits: FooString | None = None
+    crew: FooString | None = None
+    films: list[FooFilm]
+    films_ids: list[FooID]
+    hyperdrive_rating: FooString | None = None
+    id: FooID
+    length: FooString | None = None
+    manufacturer: FooString | None = None
+    max_atmosphering_speed: FooString | None = None
+    model: FooString | None = None
+    name: FooString
+    passengers: FooString | None = None
+    pilots: list[FooPerson]
+    pilots_ids: list[FooID]
+    starship_class: FooString | None = None
+    typename__: Literal['Starship'] | None = Field('Starship', alias='__typename')
+
+
+class FooVehicle(BaseModel):
+    cargo_capacity: FooString | None = None
+    consumables: FooString | None = None
+    cost_in_credits: FooString | None = None
+    crew: FooString | None = None
+    films: list[FooFilm]
+    films_ids: list[FooID]
+    id: FooID
+    length: FooString | None = None
+    manufacturer: FooString | None = None
+    max_atmosphering_speed: FooString | None = None
+    model: FooString | None = None
+    name: FooString
+    passengers: FooString | None = None
+    pilots: list[FooPerson]
+    pilots_ids: list[FooID]
+    vehicle_class: FooString | None = None
+    typename__: Literal['Vehicle'] | None = Field('Vehicle', alias='__typename')
+
+
+FooFilm.update_forward_refs()
+FooPerson.update_forward_refs()
diff --git a/tests/data/expected/parser/graphql/union_with_prefix.py b/tests/data/expected/parser/graphql/union_with_prefix.py
new file mode 100644
index 000000000..5bee2bab4
--- /dev/null
+++ b/tests/data/expected/parser/graphql/union_with_prefix.py
@@ -0,0 +1,59 @@
+# generated by datamodel-codegen:
+#   filename:  union.graphql
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from typing import Literal, TypeAlias, Union
+
+from pydantic import BaseModel, Field
+
+FooBoolean: TypeAlias = bool
+"""
+The `Boolean` scalar type represents `true` or `false`.
+"""
+
+
+FooID: TypeAlias = str
+"""
+The `ID` scalar type represents a unique identifier, often used to refetch an object or as key for a cache. The ID type appears in a JSON response as a String; however, it is not intended to be human-readable. When expected as an input type, any string (such as `"4"`) or integer (such as `4`) input value will be accepted as an ID.
+"""
+
+
+FooInt: TypeAlias = int
+"""
+The `Int` scalar type represents non-fractional signed whole numeric values. Int can represent values between -(2^31) and 2^31 - 1.
+"""
+
+
+FooString: TypeAlias = str
+"""
+The `String` scalar type represents textual data, represented as UTF-8 character sequences. The String type is most often used by GraphQL to represent free-form human-readable text.
+"""
+
+
+class FooIResource(BaseModel):
+    id: FooID
+    typename__: Literal['IResource'] | None = Field('IResource', alias='__typename')
+
+
+class FooCar(FooIResource):
+    id: FooID
+    passengerCapacity: FooInt
+    typename__: Literal['Car'] | None = Field('Car', alias='__typename')
+
+
+class FooEmployee(FooIResource):
+    firstName: FooString | None = None
+    id: FooID
+    lastName: FooString | None = None
+    typename__: Literal['Employee'] | None = Field('Employee', alias='__typename')
+
+
+FooResource: TypeAlias = Union[
+    'FooCar',
+    'FooEmployee',
+]
+
+
+FooTechnicalResource: TypeAlias = FooCar
diff --git a/tests/main/graphql/test_main_graphql.py b/tests/main/graphql/test_main_graphql.py
index 0ed095fd9..c33914869 100644
--- a/tests/main/graphql/test_main_graphql.py
+++ b/tests/main/graphql/test_main_graphql.py
@@ -762,6 +762,21 @@ def test_main_graphql_dataclass_frozen_keyword_only(output_file: Path) -> None:
     )
 
 
+def test_main_graphql_class_name_prefix(output_file: Path) -> None:
+    """Test GraphQL code generation with class name prefixing."""
+    run_main_and_assert(
+        input_path=GRAPHQL_DATA_PATH / "simple-star-wars.graphql",
+        output_path=output_file,
+        input_file_type="graphql",
+        assert_func=assert_file_content,
+        expected_file="simple_star_wars_class_name_prefix.py",
+        extra_args=[
+            "--class-name-prefix",
+            "Foo",
+        ],
+    )
+
+
 def test_main_graphql_union_snake_case_field(output_file: Path) -> None:
     """Test that union type references are not converted to snake_case."""
     run_main_and_assert(
diff --git a/tests/parser/test_graphql.py b/tests/parser/test_graphql.py
index 5c236bd45..af8825133 100644
--- a/tests/parser/test_graphql.py
+++ b/tests/parser/test_graphql.py
@@ -55,6 +55,18 @@ def test_graphql_union_commented(output_file: Path) -> None:
     )
 
 
+def test_graphql_union_with_prefix(output_file: Path) -> None:
+    """Test parsing GraphQL union with class name prefix (Unions should reference prefixed class names)."""
+    run_main_and_assert(
+        input_path=GRAPHQL_DATA_PATH / "union.graphql",
+        output_path=output_file,
+        input_file_type="graphql",
+        assert_func=assert_file_content,
+        expected_file="union_with_prefix.py",
+        extra_args=["--class-name-prefix", "Foo"],
+    )
+
+
 @pytest.mark.parametrize(
     ("frozen_dataclasses", "keyword_only", "parser_dataclass_args", "kwargs_dataclass_args", "expected"),
     [