diff --git a/packages/google-auth/google/auth/compute_engine/_metadata.py b/packages/google-auth/google/auth/compute_engine/_metadata.py
index c1fecd3b7dca..aae724ab18ee 100644
--- a/packages/google-auth/google/auth/compute_engine/_metadata.py
+++ b/packages/google-auth/google/auth/compute_engine/_metadata.py
@@ -166,16 +166,15 @@ def _prepare_request_for_mds(request, use_mtls=False) -> None:
 
     Args:
         request (google.auth.transport.Request): A callable used to make
-            HTTP requests.
+            HTTP requests. If mTLS is enabled, and the request supports sessions,
+            the request will have the mTLS adapter mounted. Otherwise, there
+            will be no change.
         use_mtls (bool): Whether to use mTLS for the request.
 
-    Returns:
-        google.auth.transport.Request: A request object to use.
-            If mTLS is enabled, the request will have the mTLS adapter mounted.
-            Otherwise, the original request will be returned unchanged.
+
     """
-    # Only modify the request if mTLS is enabled.
-    if use_mtls:
+    # Only modify the request if mTLS is enabled, and request supports sessions.
+    if use_mtls and hasattr(request, "session"):
         # Ensure the request has a session to mount the adapter to.
         if not request.session:
             request.session = requests.Session()
diff --git a/packages/google-auth/tests/compute_engine/test__metadata.py b/packages/google-auth/tests/compute_engine/test__metadata.py
index 9fc2c8cf6cb5..35996ab24b92 100644
--- a/packages/google-auth/tests/compute_engine/test__metadata.py
+++ b/packages/google-auth/tests/compute_engine/test__metadata.py
@@ -955,3 +955,17 @@ def test__prepare_request_for_mds_mtls_no_session(mock_mds_mtls_adapter):
         mock_session_class.assert_called_once()
         mock_mds_mtls_adapter.assert_called_once()
         assert request.session.mount.call_count == len(_metadata._GCE_DEFAULT_MDS_HOSTS)
+
+
+@mock.patch("google.auth.compute_engine._mtls.MdsMtlsAdapter")
+def test__prepare_request_for_mds_mtls_http_request(mock_mds_mtls_adapter):
+    """
+    http requests should be ignored.
+    Regression test for https://github.com/googleapis/google-cloud-python/issues/16035
+    """
+    from google.auth.transport import _http_client
+
+    request = _http_client.Request()
+    _metadata._prepare_request_for_mds(request, use_mtls=True)
+
+    assert mock_mds_mtls_adapter.call_count == 0