diff --git a/google/auth/iam.py b/google/auth/iam.py
index bd0500457..d83b25180 100644
--- a/google/auth/iam.py
+++ b/google/auth/iam.py
@@ -69,7 +69,7 @@ def _make_signing_request(self, message):
 
         method = "POST"
         url = _SIGN_BLOB_URI.format(self._service_account_email)
-        headers = {}
+        headers = {"Content-Type": "application/json"}
         body = json.dumps(
             {"bytesToSign": base64.b64encode(message).decode("utf-8")}
         ).encode("utf-8")
diff --git a/tests/test_iam.py b/tests/test_iam.py
index 4367fe7a8..cb2c26f73 100644
--- a/tests/test_iam.py
+++ b/tests/test_iam.py
@@ -89,6 +89,8 @@ def test_sign_bytes(self):
         returned_signature = signer.sign("123")
 
         assert returned_signature == signature
+        kwargs = request.call_args.kwargs
+        assert kwargs["headers"]["Content-Type"] == "application/json"
 
     def test_sign_bytes_failure(self):
         request = make_request(http_client.UNAUTHORIZED)