diff --git a/google/auth/transport/requests.py b/google/auth/transport/requests.py
index 2268243..8250c74 100644
--- a/google/auth/transport/requests.py
+++ b/google/auth/transport/requests.py
@@ -150,33 +150,40 @@
             refresh the credentials and retry the request.
         refresh_timeout (Optional[int]): The timeout value in seconds for
             credential refresh HTTP requests.
-        kwargs: Additional arguments passed to the :class:`requests.Session`
-            constructor.
+        auth_request (google.auth.transport.requests.Request):
+            (Optional) An instance of
+            :class:`~google.auth.transport.requests.Request` used when
+            refreshing credentials. If not passed,
+            an instance of :class:`~google.auth.transport.requests.Request`
+            is created.
     """
     def __init__(self, credentials,
                  refresh_status_codes=transport.DEFAULT_REFRESH_STATUS_CODES,
                  max_refresh_attempts=transport.DEFAULT_MAX_REFRESH_ATTEMPTS,
                  refresh_timeout=None,
-                 **kwargs):
-        super(AuthorizedSession, self).__init__(**kwargs)
+                 auth_request=None):
+        super(AuthorizedSession, self).__init__()
         self.credentials = credentials
         self._refresh_status_codes = refresh_status_codes
         self._max_refresh_attempts = max_refresh_attempts
         self._refresh_timeout = refresh_timeout
 
-        auth_request_session = requests.Session()
+        if auth_request is None:
+            auth_request_session = requests.Session()
 
-        # Using an adapter to make HTTP requests robust to network errors.
-        # This adapter retrys HTTP requests when network errors occur
-        # and the requests seems safely retryable.
-        retry_adapter = requests.adapters.HTTPAdapter(max_retries=3)
-        auth_request_session.mount("https://", retry_adapter)
+            # Using an adapter to make HTTP requests robust to network errors.
+            # This adapter retrys HTTP requests when network errors occur
+            # and the requests seems safely retryable.
+            retry_adapter = requests.adapters.HTTPAdapter(max_retries=3)
+            auth_request_session.mount("https://", retry_adapter)
+
+            # Do not pass `self` as the session here, as it can lead to
+            # infinite recursion.
+            auth_request = Request(auth_request_session)
 
         # Request instance used by internal methods (for example,
         # credentials.refresh).
-        # Do not pass `self` as the session here, as it can lead to infinite
-        # recursion.
-        self._auth_request = Request(auth_request_session)
+        self._auth_request = auth_request
 
     def request(self, method, url, data=None, headers=None, **kwargs):
         """Implementation of Requests' request."""
diff --git a/tests/transport/test_requests.py b/tests/transport/test_requests.py
index 41dc237..311992a 100644
--- a/tests/transport/test_requests.py
+++ b/tests/transport/test_requests.py
@@ -85,6 +85,15 @@
 
         assert authed_session.credentials == mock.sentinel.credentials
 
+    def test_constructor_with_auth_request(self):
+        http = mock.create_autospec(requests.Session)
+        auth_request = google.auth.transport.requests.Request(http)
+
+        authed_session = google.auth.transport.requests.AuthorizedSession(
+            mock.sentinel.credentials, auth_request=auth_request)
+
+        assert authed_session._auth_request == auth_request
+
     def test_request_no_refresh(self):
         credentials = mock.Mock(wraps=CredentialsStub())
         response = make_response()