diff --git a/packages/google-auth/setup.py b/packages/google-auth/setup.py
index a9fe43da8468..1dfb9ee734ff 100644
--- a/packages/google-auth/setup.py
+++ b/packages/google-auth/setup.py
@@ -30,7 +30,11 @@
 
 requests_extra_require = ["requests >= 2.30.0, < 3.0.0"]
 
-aiohttp_extra_require = ["aiohttp >= 3.8.0, < 4.0.0", *requests_extra_require]
+aiohttp_extra_require = [
+    "aiohttp >= 3.8.0, < 4.0.0; python_version < '3.14'",
+    "aiohttp >= 3.9.0, < 4.0.0; python_version >= '3.14'",
+    *requests_extra_require,
+]
 
 pyjwt_extra_require = ["pyjwt>=2.0"]
 
diff --git a/packages/google-auth/testing/constraints-3.14.txt b/packages/google-auth/testing/constraints-3.14.txt
index abdcf9de1e42..64212e65e4df 100644
--- a/packages/google-auth/testing/constraints-3.14.txt
+++ b/packages/google-auth/testing/constraints-3.14.txt
@@ -1,4 +1,10 @@
-# Lower-bound constraints for Python 3.14 core dependencies.
-# Used during CI unit tests to verify minimum supported package versions.
+# This constraints file is used to check that lower bounds
+# are correct in setup.py
+# List *all* library dependencies and extras in this file.
+# Pin the version to the lower bound.
+#
+# e.g., if setup.py has "foo >= 1.14.0, < 2.0.0dev",
+# Then this file should have foo==1.14.0
 pyasn1-modules==0.2.1
 cryptography==41.0.5
+aiohttp==3.9.0
diff --git a/packages/google-auth/tests/transport/aio/conftest.py b/packages/google-auth/tests/transport/aio/conftest.py
index 6b415af5aaf0..126819af6a45 100644
--- a/packages/google-auth/tests/transport/aio/conftest.py
+++ b/packages/google-auth/tests/transport/aio/conftest.py
@@ -12,35 +12,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import inspect
-from unittest.mock import Mock
+from tests.transport.aioresponses_compat import patch_aioresponses
 
-import aiohttp
-from aioresponses.core import RequestMatch  # type: ignore
-
-
-class _CompatClientResponse(aiohttp.ClientResponse):
-    """ClientResponse subclass for aioresponses compatibility across all aiohttp versions."""
-
-    def __init__(self, *args, **kwargs):
-        writer = kwargs.pop("writer", None)
-        stream_writer = kwargs.pop("stream_writer", None)
-        writer_obj = stream_writer or writer or Mock()
-        sig = inspect.signature(super().__init__)
-        if "stream_writer" in sig.parameters:
-            kwargs["stream_writer"] = writer_obj
-        if "writer" in sig.parameters:
-            kwargs["writer"] = writer_obj
-        super().__init__(*args, **kwargs)
-
-
-_orig_request_match_init = RequestMatch.__init__
-
-
-def _request_match_init(self, *args, **kwargs):
-    if kwargs.get("response_class") is None:
-        kwargs["response_class"] = _CompatClientResponse
-    _orig_request_match_init(self, *args, **kwargs)
-
-
-RequestMatch.__init__ = _request_match_init
+patch_aioresponses()
diff --git a/packages/google-auth/tests/transport/aioresponses_compat.py b/packages/google-auth/tests/transport/aioresponses_compat.py
new file mode 100644
index 000000000000..84493d7ffe3d
--- /dev/null
+++ b/packages/google-auth/tests/transport/aioresponses_compat.py
@@ -0,0 +1,66 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import inspect
+
+import aiohttp
+from aioresponses.core import RequestMatch  # type: ignore
+
+
+class _DummyWriter:
+    output_size = 0
+
+    def done(self):
+        return True
+
+    def add_done_callback(self, fn, *args, **kwargs):
+        pass
+
+    def remove_done_callback(self, fn):
+        pass
+
+    def __await__(self):
+        if False:
+            yield
+        return None
+
+
+class _CompatClientResponse(aiohttp.ClientResponse):
+    """ClientResponse subclass for aioresponses compatibility across all aiohttp versions."""
+
+    def __init__(self, *args, **kwargs):
+        writer = kwargs.pop("writer", None)
+        stream_writer = kwargs.pop("stream_writer", None)
+        writer_obj = stream_writer or writer or _DummyWriter()
+        sig = inspect.signature(super().__init__)
+        if "stream_writer" in sig.parameters:
+            kwargs["stream_writer"] = writer_obj
+        if "writer" in sig.parameters:
+            kwargs["writer"] = writer_obj
+        super().__init__(*args, **kwargs)
+
+
+def patch_aioresponses():
+    """Patch RequestMatch.__init__ to use _CompatClientResponse by default."""
+    if getattr(RequestMatch, "_is_compat_patched", False):
+        return
+    orig_init = RequestMatch.__init__
+
+    def _request_match_init(self, *args, **kwargs):
+        if kwargs.get("response_class") is None:
+            kwargs["response_class"] = _CompatClientResponse
+        orig_init(self, *args, **kwargs)
+
+    RequestMatch.__init__ = _request_match_init
+    RequestMatch._is_compat_patched = True
diff --git a/packages/google-auth/tests_async/transport/conftest.py b/packages/google-auth/tests_async/transport/conftest.py
index 6b415af5aaf0..126819af6a45 100644
--- a/packages/google-auth/tests_async/transport/conftest.py
+++ b/packages/google-auth/tests_async/transport/conftest.py
@@ -12,35 +12,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import inspect
-from unittest.mock import Mock
+from tests.transport.aioresponses_compat import patch_aioresponses
 
-import aiohttp
-from aioresponses.core import RequestMatch  # type: ignore
-
-
-class _CompatClientResponse(aiohttp.ClientResponse):
-    """ClientResponse subclass for aioresponses compatibility across all aiohttp versions."""
-
-    def __init__(self, *args, **kwargs):
-        writer = kwargs.pop("writer", None)
-        stream_writer = kwargs.pop("stream_writer", None)
-        writer_obj = stream_writer or writer or Mock()
-        sig = inspect.signature(super().__init__)
-        if "stream_writer" in sig.parameters:
-            kwargs["stream_writer"] = writer_obj
-        if "writer" in sig.parameters:
-            kwargs["writer"] = writer_obj
-        super().__init__(*args, **kwargs)
-
-
-_orig_request_match_init = RequestMatch.__init__
-
-
-def _request_match_init(self, *args, **kwargs):
-    if kwargs.get("response_class") is None:
-        kwargs["response_class"] = _CompatClientResponse
-    _orig_request_match_init(self, *args, **kwargs)
-
-
-RequestMatch.__init__ = _request_match_init
+patch_aioresponses()