diff --git a/google/auth/_default.py b/google/auth/_default.py
index 4dc0725..f7e308f 100644
--- a/google/auth/_default.py
+++ b/google/auth/_default.py
@@ -230,6 +230,11 @@
 
 def _get_gae_credentials():
     """Gets Google App Engine App Identity credentials and project ID."""
+    # If not GAE gen1, prefer the metadata service even if the GAE APIs are
+    # available as per https://google.aip.dev/auth/4115.
+    if os.environ.get(environment_vars.LEGACY_APPENGINE_RUNTIME) != "python27":
+        return None, None
+
     # While this library is normally bundled with app_engine, there are
     # some cases where it's not available, so we tolerate ImportError.
     try:
diff --git a/google/auth/environment_vars.py b/google/auth/environment_vars.py
index f027741..d36d6c4 100644
--- a/google/auth/environment_vars.py
+++ b/google/auth/environment_vars.py
@@ -60,6 +60,12 @@
 The default value is false. Users have to explicitly set this value to true
 in order to use client certificate to establish a mutual TLS channel."""
 
+LEGACY_APPENGINE_RUNTIME = "APPENGINE_RUNTIME"
+"""Gen1 environment variable defining the App Engine Runtime.
+
+Used to distinguish between GAE gen1 and GAE gen2+.
+"""
+
 # AWS environment variables used with AWS workload identity pools to retrieve
 # AWS security credentials and the AWS region needed to create a serialized
 # signed requests to the AWS STS GetCalledIdentity API that can be exchanged
diff --git a/tests/test__default.py b/tests/test__default.py
index e136896..a515f38 100644
--- a/tests/test__default.py
+++ b/tests/test__default.py
@@ -447,7 +447,9 @@
     yield app_identity_module
 
 
-def test__get_gae_credentials(app_identity):
+@mock.patch.dict(os.environ)
+def test__get_gae_credentials_gen1(app_identity):
+    os.environ[environment_vars.LEGACY_APPENGINE_RUNTIME] = "python27"
     app_identity.get_application_id.return_value = mock.sentinel.project
 
     credentials, project_id = _default._get_gae_credentials()
@@ -456,18 +458,65 @@
     assert project_id == mock.sentinel.project
 
 
+@mock.patch.dict(os.environ)
+def test__get_gae_credentials_gen2():
+    os.environ["GAE_RUNTIME"] = "python37"
+    credentials, project_id = _default._get_gae_credentials()
+    assert credentials is None
+    assert project_id is None
+
+
+@mock.patch.dict(os.environ)
+def test__get_gae_credentials_gen2_backwards_compat():
+    # compat helpers may copy GAE_RUNTIME to APPENGINE_RUNTIME
+    # for backwards compatibility with code that relies on it
+    os.environ[environment_vars.LEGACY_APPENGINE_RUNTIME] = "python37"
+    os.environ["GAE_RUNTIME"] = "python37"
+    credentials, project_id = _default._get_gae_credentials()
+    assert credentials is None
+    assert project_id is None
+
+
+def test__get_gae_credentials_env_unset():
+    assert environment_vars.LEGACY_APPENGINE_RUNTIME not in os.environ
+    assert "GAE_RUNTIME" not in os.environ
+    credentials, project_id = _default._get_gae_credentials()
+    assert credentials is None
+    assert project_id is None
+
+
+@mock.patch.dict(os.environ)
 def test__get_gae_credentials_no_app_engine():
+    # test both with and without LEGACY_APPENGINE_RUNTIME setting
+    assert environment_vars.LEGACY_APPENGINE_RUNTIME not in os.environ
+
     import sys
 
-    with mock.patch.dict("sys.modules"):
-        sys.modules["google.auth.app_engine"] = None
+    with mock.patch.dict(sys.modules, {"google.auth.app_engine": None}):
+        credentials, project_id = _default._get_gae_credentials()
+        assert credentials is None
+        assert project_id is None
+
+        os.environ[environment_vars.LEGACY_APPENGINE_RUNTIME] = "python27"
         credentials, project_id = _default._get_gae_credentials()
         assert credentials is None
         assert project_id is None
 
 
+@mock.patch.dict(os.environ)
+@mock.patch.object(app_engine, "app_identity", new=None)
 def test__get_gae_credentials_no_apis():
-    assert _default._get_gae_credentials() == (None, None)
+    # test both with and without LEGACY_APPENGINE_RUNTIME setting
+    assert environment_vars.LEGACY_APPENGINE_RUNTIME not in os.environ
+
+    credentials, project_id = _default._get_gae_credentials()
+    assert credentials is None
+    assert project_id is None
+
+    os.environ[environment_vars.LEGACY_APPENGINE_RUNTIME] = "python27"
+    credentials, project_id = _default._get_gae_credentials()
+    assert credentials is None
+    assert project_id is None
 
 
 @mock.patch(
diff --git a/tests/test_app_engine.py b/tests/test_app_engine.py
index e335ff7..6a788b9 100644
--- a/tests/test_app_engine.py
+++ b/tests/test_app_engine.py
@@ -52,6 +52,7 @@
     assert app_engine.get_project_id() == mock.sentinel.project
 
 
+@mock.patch.object(app_engine, "app_identity", new=None)
 def test_get_project_id_missing_apis():
     with pytest.raises(EnvironmentError) as excinfo:
         assert app_engine.get_project_id()
@@ -86,6 +87,7 @@
 
 
 class TestCredentials(object):
+    @mock.patch.object(app_engine, "app_identity", new=None)
     def test_missing_apis(self):
         with pytest.raises(EnvironmentError) as excinfo:
             app_engine.Credentials()
diff --git a/tests_async/test__default_async.py b/tests_async/test__default_async.py
index 527a8da..b672303 100644
--- a/tests_async/test__default_async.py
+++ b/tests_async/test__default_async.py
@@ -284,7 +284,9 @@
     yield app_identity_module
 
 
-def test__get_gae_credentials(app_identity):
+@mock.patch.dict(os.environ)
+def test__get_gae_credentials_gen1(app_identity):
+    os.environ[environment_vars.LEGACY_APPENGINE_RUNTIME] = "python27"
     app_identity.get_application_id.return_value = mock.sentinel.project
 
     credentials, project_id = _default._get_gae_credentials()
@@ -293,18 +295,65 @@
     assert project_id == mock.sentinel.project
 
 
+@mock.patch.dict(os.environ)
+def test__get_gae_credentials_gen2():
+    os.environ["GAE_RUNTIME"] = "python37"
+    credentials, project_id = _default._get_gae_credentials()
+    assert credentials is None
+    assert project_id is None
+
+
+@mock.patch.dict(os.environ)
+def test__get_gae_credentials_gen2_backwards_compat():
+    # compat helpers may copy GAE_RUNTIME to APPENGINE_RUNTIME
+    # for backwards compatibility with code that relies on it
+    os.environ[environment_vars.LEGACY_APPENGINE_RUNTIME] = "python37"
+    os.environ["GAE_RUNTIME"] = "python37"
+    credentials, project_id = _default._get_gae_credentials()
+    assert credentials is None
+    assert project_id is None
+
+
+def test__get_gae_credentials_env_unset():
+    assert environment_vars.LEGACY_APPENGINE_RUNTIME not in os.environ
+    assert "GAE_RUNTIME" not in os.environ
+    credentials, project_id = _default._get_gae_credentials()
+    assert credentials is None
+    assert project_id is None
+
+
+@mock.patch.dict(os.environ)
 def test__get_gae_credentials_no_app_engine():
+    # test both with and without LEGACY_APPENGINE_RUNTIME setting
+    assert environment_vars.LEGACY_APPENGINE_RUNTIME not in os.environ
+
     import sys
 
-    with mock.patch.dict("sys.modules"):
-        sys.modules["google.auth.app_engine"] = None
+    with mock.patch.dict(sys.modules, {"google.auth.app_engine": None}):
+        credentials, project_id = _default._get_gae_credentials()
+        assert credentials is None
+        assert project_id is None
+
+        os.environ[environment_vars.LEGACY_APPENGINE_RUNTIME] = "python27"
         credentials, project_id = _default._get_gae_credentials()
         assert credentials is None
         assert project_id is None
 
 
+@mock.patch.dict(os.environ)
+@mock.patch.object(app_engine, "app_identity", new=None)
 def test__get_gae_credentials_no_apis():
-    assert _default._get_gae_credentials() == (None, None)
+    # test both with and without LEGACY_APPENGINE_RUNTIME setting
+    assert environment_vars.LEGACY_APPENGINE_RUNTIME not in os.environ
+
+    credentials, project_id = _default._get_gae_credentials()
+    assert credentials is None
+    assert project_id is None
+
+    os.environ[environment_vars.LEGACY_APPENGINE_RUNTIME] = "python27"
+    credentials, project_id = _default._get_gae_credentials()
+    assert credentials is None
+    assert project_id is None
 
 
 @mock.patch(