diff --git a/google/auth/iam.py b/google/auth/iam.py
index bba1624c1..1c2eb913b 100644
--- a/google/auth/iam.py
+++ b/google/auth/iam.py
@@ -23,10 +23,18 @@
 import http.client as http_client
 import json
 
+from google.auth import _exponential_backoff
 from google.auth import _helpers
 from google.auth import crypt
 from google.auth import exceptions
 
+IAM_RETRY_CODES = {
+    http_client.INTERNAL_SERVER_ERROR,
+    http_client.BAD_GATEWAY,
+    http_client.SERVICE_UNAVAILABLE,
+    http_client.GATEWAY_TIMEOUT,
+}
+
 
 _IAM_SCOPE = ["https://www.googleapis.com/auth/iam"]
 
@@ -88,15 +96,22 @@ def _make_signing_request(self, message):
             {"payload": base64.b64encode(message).decode("utf-8")}
         ).encode("utf-8")
 
-        self._credentials.before_request(self._request, method, url, headers)
-        response = self._request(url=url, method=method, body=body, headers=headers)
+        retries = _exponential_backoff.ExponentialBackoff()
+        for _ in retries:
+            self._credentials.before_request(self._request, method, url, headers)
+
+            response = self._request(url=url, method=method, body=body, headers=headers)
+
+            if response.status in IAM_RETRY_CODES:
+                continue
 
-        if response.status != http_client.OK:
-            raise exceptions.TransportError(
-                "Error calling the IAM signBlob API: {}".format(response.data)
-            )
+            if response.status != http_client.OK:
+                raise exceptions.TransportError(
+                    "Error calling the IAM signBlob API: {}".format(response.data)
+                )
 
-        return json.loads(response.data.decode("utf-8"))
+            return json.loads(response.data.decode("utf-8"))
+        raise exceptions.TransportError("exhausted signBlob endpoint retries")
 
     @property
     def key_id(self):
diff --git a/google/auth/impersonated_credentials.py b/google/auth/impersonated_credentials.py
index c42a93643..afac4120b 100644
--- a/google/auth/impersonated_credentials.py
+++ b/google/auth/impersonated_credentials.py
@@ -31,6 +31,7 @@
 import http.client as http_client
 import json
 
+from google.auth import _exponential_backoff
 from google.auth import _helpers
 from google.auth import credentials
 from google.auth import exceptions
@@ -288,18 +289,22 @@ def sign_bytes(self, message):
         authed_session = AuthorizedSession(self._source_credentials)
 
         try:
-            response = authed_session.post(
-                url=iam_sign_endpoint, headers=headers, json=body
-            )
+            retries = _exponential_backoff.ExponentialBackoff()
+            for _ in retries:
+                response = authed_session.post(
+                    url=iam_sign_endpoint, headers=headers, json=body
+                )
+                if response.status_code in iam.IAM_RETRY_CODES:
+                    continue
+                if response.status_code != http_client.OK:
+                    raise exceptions.TransportError(
+                        "Error calling sign_bytes: {}".format(response.json())
+                    )
+
+                return base64.b64decode(response.json()["signedBlob"])
         finally:
             authed_session.close()
-
-        if response.status_code != http_client.OK:
-            raise exceptions.TransportError(
-                "Error calling sign_bytes: {}".format(response.json())
-            )
-
-        return base64.b64decode(response.json()["signedBlob"])
+        raise exceptions.TransportError("exhausted signBlob endpoint retries")
 
     @property
     def signer_email(self):
diff --git a/system_tests/secrets.tar.enc b/system_tests/secrets.tar.enc
index ebddbfe05..4a6a25883 100644
Binary files a/system_tests/secrets.tar.enc and b/system_tests/secrets.tar.enc differ
diff --git a/tests/test_iam.py b/tests/test_iam.py
index 6706afb4b..01c2fa085 100644
--- a/tests/test_iam.py
+++ b/tests/test_iam.py
@@ -91,6 +91,7 @@ def test_sign_bytes(self):
         assert returned_signature == signature
         kwargs = request.call_args[1]
         assert kwargs["headers"]["Content-Type"] == "application/json"
+        request.call_count == 1
 
     def test_sign_bytes_failure(self):
         request = make_request(http_client.UNAUTHORIZED)
@@ -100,3 +101,15 @@ def test_sign_bytes_failure(self):
 
         with pytest.raises(exceptions.TransportError):
             signer.sign("123")
+        request.call_count == 1
+
+    @mock.patch("time.sleep", return_value=None)
+    def test_sign_bytes_retryable_failure(self, mock_time):
+        request = make_request(http_client.INTERNAL_SERVER_ERROR)
+        credentials = make_credentials()
+
+        signer = iam.Signer(request, credentials, mock.sentinel.service_account_email)
+
+        with pytest.raises(exceptions.TransportError):
+            signer.sign("123")
+        request.call_count == 3
diff --git a/tests/test_impersonated_credentials.py b/tests/test_impersonated_credentials.py
index 83e260638..f467269e2 100644
--- a/tests/test_impersonated_credentials.py
+++ b/tests/test_impersonated_credentials.py
@@ -426,12 +426,28 @@ def test_sign_bytes_failure(self):
             "google.auth.transport.requests.AuthorizedSession.request", autospec=True
         ) as auth_session:
             data = {"error": {"code": 403, "message": "unauthorized"}}
-            auth_session.return_value = MockResponse(data, http_client.FORBIDDEN)
+            mock_response = MockResponse(data, http_client.UNAUTHORIZED)
+            auth_session.return_value = mock_response
 
             with pytest.raises(exceptions.TransportError) as excinfo:
                 credentials.sign_bytes(b"foo")
             assert excinfo.match("'code': 403")
 
+    @mock.patch("time.sleep", return_value=None)
+    def test_sign_bytes_retryable_failure(self, mock_time):
+        credentials = self.make_credentials(lifetime=None)
+
+        with mock.patch(
+            "google.auth.transport.requests.AuthorizedSession.request", autospec=True
+        ) as auth_session:
+            data = {"error": {"code": 500, "message": "internal_failure"}}
+            mock_response = MockResponse(data, http_client.INTERNAL_SERVER_ERROR)
+            auth_session.return_value = mock_response
+
+            with pytest.raises(exceptions.TransportError) as excinfo:
+                credentials.sign_bytes(b"foo")
+            assert excinfo.match("exhausted signBlob endpoint retries")
+
     def test_with_quota_project(self):
         credentials = self.make_credentials()