diff --git a/packages/gapic-generator/noxfile.py b/packages/gapic-generator/noxfile.py
index 6fac5c48853d..343626eab4e4 100644
--- a/packages/gapic-generator/noxfile.py
+++ b/packages/gapic-generator/noxfile.py
@@ -457,7 +457,7 @@ def showcase_mtls(
     """Run the Showcase mtls test suite."""
 
     with showcase_library(session, templates=templates, other_opts=other_opts):
-        session.install("pytest", "pytest-asyncio")
+        session.install("pytest", "pytest-asyncio", "pyopenssl")
         test_directory = Path("tests", "system")
         ignore_file = env.get("IGNORE_FILE")
         pytest_command = [
diff --git a/packages/gapic-generator/tests/system/conftest.py b/packages/gapic-generator/tests/system/conftest.py
index 180e48b8d59a..5c00c99a5976 100644
--- a/packages/gapic-generator/tests/system/conftest.py
+++ b/packages/gapic-generator/tests/system/conftest.py
@@ -18,6 +18,8 @@
 import os
 import pytest
 import pytest_asyncio
+from requests.adapters import HTTPAdapter
+from urllib3.poolmanager import PoolManager
 
 from typing import Sequence, Tuple
 
@@ -328,7 +330,7 @@ def _read_response_metadata_stream(self):
     def intercept_unary_unary(self, continuation, client_call_details, request):
         self._add_request_metadata(client_call_details)
         response = continuation(client_call_details, request)
-        metadata = [(k, str(v)) for k, v in response.trailing_metadata()]
+        metadata = [(k, str(v)) for k, v in response.initial_metadata()] + [(k, str(v)) for k, v in response.trailing_metadata()]
         self.response_metadata = metadata
         return response
 
@@ -453,24 +455,44 @@ async def intercepted_echo_grpc_async():
     return EchoAsyncClient(transport=transport), interceptor
 
 
+class HostNameIgnoringAdapter(HTTPAdapter):
+    """Custom HTTPAdapter that disables hostname verification for local self-signed certs."""
+    def init_poolmanager(self, connections, maxsize, block=False, **pool_kwargs):
+        self.poolmanager = PoolManager(
+            num_pools=connections,
+            maxsize=maxsize,
+            block=block,
+            assert_hostname=False,
+            **pool_kwargs
+        )
+
+
 @pytest.fixture
-def intercepted_echo_rest():
+def intercepted_echo_rest(use_mtls):
     transport_name = "rest"
     transport_cls = EchoClient.get_transport_class(transport_name)
     interceptor = EchoMetadataClientRestInterceptor()
 
-    # The custom host explicitly bypasses https.
+    url_scheme = "https" if use_mtls else "http"
     transport = transport_cls(
         credentials=ga_credentials.AnonymousCredentials(),
         host="localhost:7469",
-        url_scheme="http",
+        url_scheme=url_scheme,
         interceptor=interceptor,
     )
+    if use_mtls:
+        dir = os.path.dirname(__file__)
+        cert_path = os.path.join(dir, "../cert/mtls.crt")
+        key_path = os.path.join(dir, "../cert/mtls.key")
+        transport._session.verify = cert_path
+        transport._session.cert = (cert_path, key_path)
+        transport._session.mount("https://", HostNameIgnoringAdapter())
+
     return EchoClient(transport=transport), interceptor
 
 
 @pytest.fixture
-def intercepted_echo_rest_async():
+def intercepted_echo_rest_async(use_mtls):
     if not HAS_ASYNC_REST_ECHO_TRANSPORT:
         pytest.skip("Skipping test with async rest.")
 
@@ -478,11 +500,19 @@ def intercepted_echo_rest_async():
     transport_cls = EchoAsyncClient.get_transport_class(transport_name)
     interceptor = EchoMetadataClientRestAsyncInterceptor()
 
-    # The custom host explicitly bypasses https.
+    url_scheme = "https" if use_mtls else "http"
     transport = transport_cls(
         credentials=async_anonymous_credentials(),
         host="localhost:7469",
-        url_scheme="http",
+        url_scheme=url_scheme,
         interceptor=interceptor,
     )
+    if use_mtls:
+        dir = os.path.dirname(__file__)
+        cert_path = os.path.join(dir, "../cert/mtls.crt")
+        key_path = os.path.join(dir, "../cert/mtls.key")
+        transport._session.verify = cert_path
+        transport._session.cert = (cert_path, key_path)
+        transport._session.mount("https://", HostNameIgnoringAdapter())
+
     return EchoAsyncClient(transport=transport), interceptor
diff --git a/packages/gapic-generator/tests/system/test_pqc.py b/packages/gapic-generator/tests/system/test_pqc.py
new file mode 100644
index 000000000000..6b6876272c65
--- /dev/null
+++ b/packages/gapic-generator/tests/system/test_pqc.py
@@ -0,0 +1,98 @@
+# 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
+#
+#     https://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 pytest
+import requests
+from google import showcase
+
+
+@pytest.fixture
+def run_pqc_test(use_mtls):
+    if not use_mtls:
+        pytest.skip("PQC integration test requires mTLS (--mtls flag) to be enabled.")
+
+
+@pytest.mark.parametrize(
+    "transport_fixture",
+    ["intercepted_echo_grpc", "intercepted_echo_rest"]
+)
+def test_pqc_negotiated_group(run_pqc_test, request, transport_fixture):
+    """Verifies that the generated client library negotiates PQC (X25519MLKEM768) with Showcase server."""
+    client, interceptor = request.getfixturevalue(transport_fixture)
+
+    # Make secure call using standard GAPIC client library fixture
+    response = client.echo(request=showcase.EchoRequest(content="Verify PQC connection."))
+    assert response.content == "Verify PQC connection."
+
+    # Extract negotiated group and supported groups from response headers
+    negotiated_group = None
+    supported_groups = None
+    for key, value in interceptor.response_metadata:
+        if key.lower() == "x-showcase-tls-group":
+            negotiated_group = value
+        elif key.lower() == "x-showcase-tls-client-supported-groups":
+            supported_groups = value
+
+    assert negotiated_group is not None, "Failed: Showcase server did not return negotiated TLS group header."
+    assert supported_groups is not None, "Failed: Showcase server did not return client advertised supported groups."
+
+    print(f"\n[PQC Verification] ({transport_fixture}) Negotiated TLS Group: {negotiated_group}")
+    print(f"[PQC Verification] ({transport_fixture}) Client Advertised Supported Groups: {supported_groups}")
+
+    # Enforce PQC compliance (X25519MLKEM768 or Kyber)
+    assert "MLKEM" in negotiated_group or "Kyber" in negotiated_group, \
+        f"Failed: {transport_fixture} Connection is NOT PQC-compliant! Negotiated: {negotiated_group}"
+
+
+def test_google_auth_transport_pqc(run_pqc_test):
+    """Verifies that google-auth transport adapter negotiates PQC (X25519MLKEM768) with Showcase server."""
+    import google.auth.transport.requests
+    from conftest import HostNameIgnoringAdapter
+
+    # 1. Initialize requests Session with mTLS certs
+    session = requests.Session()
+    cert_path = "/usr/local/google/home/omairn/git/googleapis/google-cloud-python-dev2/packages/gapic-generator/tests/cert/mtls.crt"
+    key_path = "/usr/local/google/home/omairn/git/googleapis/google-cloud-python-dev2/packages/gapic-generator/tests/cert/mtls.key"
+
+    session.verify = cert_path
+    session.cert = (cert_path, key_path)
+    session.mount("https://", HostNameIgnoringAdapter())
+
+    # 2. Wrap session in google-auth transport adapter
+    auth_transport = google.auth.transport.requests.Request(session=session)
+
+    # 3. Serialize request body
+    req = showcase.EchoRequest(content="Verify google-auth transport PQC connection.")
+    body = showcase.EchoRequest.to_json(req, including_default_value_fields=False).encode("utf-8")
+
+    # 4. Execute request through google-auth's transport layer
+    url = "https://localhost:7469/v1beta1/echo:echo"
+    headers = {
+        "Content-Type": "application/json",
+        "x-goog-api-client": "gapic/1.0 rest/1.0",
+    }
+
+    response = auth_transport(url=url, method="POST", body=body, headers=headers)
+    assert response.status == 200, f"Failed: status={response.status}, body={response.data}"
+
+    # 5. Extract and verify negotiated TLS group returned by Showcase
+    negotiated_group = response.headers.get("x-showcase-tls-group")
+    supported_groups = response.headers.get("x-showcase-tls-client-supported-groups")
+
+    assert negotiated_group is not None, "Failed: Showcase server did not return negotiated TLS group header."
+    print(f"\n[google-auth Transport PQC] Negotiated TLS Group: {negotiated_group}")
+    print(f"[google-auth Transport PQC] Client Advertised Supported Groups: {supported_groups}")
+
+    assert "MLKEM" in negotiated_group or "Kyber" in negotiated_group, \
+        f"Failed: google-auth Transport is NOT PQC-compliant! Negotiated: {negotiated_group}"
diff --git a/packages/google-auth/tests/crypt/test_pqc.py b/packages/google-auth/tests/crypt/test_pqc.py
new file mode 100644
index 000000000000..5486f47967f1
--- /dev/null
+++ b/packages/google-auth/tests/crypt/test_pqc.py
@@ -0,0 +1,44 @@
+# 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
+#
+#     https://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 cryptography
+import pytest
+from packaging.version import Version
+
+
+def test_cryptography_pqc_mlkem_support():
+    """Verifies that the cryptography package installed in google-auth supports Post-Quantum Cryptography (ML-KEM / FIPS 203)."""
+    # Attempt to import ML-KEM (FIPS 203) introduced in cryptography 44.0.0
+    try:
+        from cryptography.hazmat.primitives.asymmetric import mlkem
+    except ImportError:
+        pytest.fail(
+            f"PQC Compliance Failure: cryptography version {cryptography.__version__} does not support ML-KEM (FIPS 203). "
+            f"google-auth requires cryptography >= 44.0.0 for PQC support."
+        )
+
+    # 1. Generate ML-KEM-768 (FIPS 203) private key and derive public key
+    private_key = mlkem.MLKEM768PrivateKey.generate()
+    public_key = private_key.public_key()
+    assert isinstance(private_key, mlkem.MLKEM768PrivateKey)
+    assert isinstance(public_key, mlkem.MLKEM768PublicKey)
+
+    # 2. Perform Key Encapsulation (sender generates shared secret + ciphertext)
+    shared_secret_sender, ciphertext = public_key.encapsulate()
+    assert len(ciphertext) > 0
+    assert len(shared_secret_sender) > 0
+
+    # 3. Perform Key Decapsulation (receiver recovers shared secret from ciphertext)
+    shared_secret_receiver = private_key.decapsulate(ciphertext)
+    assert shared_secret_receiver == shared_secret_sender