diff --git a/setup.py b/setup.py
index 0cef01cd..77f67366 100644
--- a/setup.py
+++ b/setup.py
@@ -24,6 +24,7 @@
 dependencies = [
     "google-api-core[grpc] >= 1.31.5, <3.0.0dev,!=2.0.*,!=2.1.*,!=2.2.*,!=2.3.0",
     'enum34; python_version < "3.4"',
+    "protobuf<4.0.0dev",
 ]
 extras = {
     "pandas": ["pandas>=0.17.1"],