diff --git a/CHANGELOG.md b/CHANGELOG.md
index f4dde004d4c2..bd66bac41a03 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 
 ### Fixed
 
+- Fixed `config_store` on Python 3.14, where `typing.Union`/`typing.Optional` annotations could no longer be remapped in-place via the now read-only `__args__` attribute
+
 ### Security
 
 ## [2.8.0] - 2026-06-05
diff --git a/test/test_config_store.py b/test/test_config_store.py
index 9c0179b9bbdb..1ea2bf24cd24 100644
--- a/test/test_config_store.py
+++ b/test/test_config_store.py
@@ -1,4 +1,4 @@
-from typing import Any, Dict, List, Tuple
+from typing import Any, Dict, List, Optional, Tuple, Union
 
 from torch_geometric.config_store import (
     class_from_dataclass,
@@ -58,6 +58,18 @@ def test_map_annotation():
     assert map_annotation(list[int], mapping) == list[Any]
     assert map_annotation(tuple[int], mapping) == tuple[Any]
 
+    # `typing.Union`/`typing.Optional` must be rebuilt without mutating
+    # `__args__`, which is read-only on Python >= 3.14:
+    assert map_annotation(Optional[int], mapping) == Optional[Any]
+    assert map_annotation(Union[int, str], mapping) == Union[Any, str]
+    assert map_annotation(List[Optional[int]], mapping) == List[Optional[Any]]
+    assert map_annotation(Dict[str, Optional[int]],
+                          mapping) == Dict[str, Optional[Any]]
+    # When every Union member maps to the same type, the rebuilt annotation
+    # collapses to canonical `Any` (the old in-place mutation left a
+    # degenerate `Union[Any, Any]`):
+    assert map_annotation(Union[int, str], {int: Any, str: Any}) == Any
+
 
 def test_register():
     register(AddSelfLoops, group='transform')
diff --git a/torch_geometric/config_store.py b/torch_geometric/config_store.py
index b45a267abd41..df6cb2f105c4 100644
--- a/torch_geometric/config_store.py
+++ b/torch_geometric/config_store.py
@@ -1,4 +1,3 @@
-import copy
 import inspect
 import typing
 from collections import defaultdict
@@ -170,10 +169,17 @@ def map_annotation(
         if type(annotation).__name__ == 'GenericAlias':
             # If annotated with `list[...]` or `dict[...]`:
             annotation = origin[args]
+        elif origin is Union:
+            # If annotated with `typing.Union[...]` or `typing.Optional[...]`.
+            # Rebuild instead of mutating `__args__`, which is read-only on
+            # Python >= 3.14 (`typing.Union` instances no longer allow
+            # in-place attribute assignment).
+            annotation = Union[args]
         else:
-            # If annotated with `typing.List[...]` or `typing.Dict[...]`:
-            annotation = copy.copy(annotation)
-            annotation.__args__ = args
+            # If annotated with `typing.List[...]` or `typing.Dict[...]`.
+            # Rebuild via `copy_with` rather than mutating `__args__`, which is
+            # read-only on Python >= 3.14.
+            annotation = annotation.copy_with(tuple(args))
 
         return annotation