diff --git a/setup.py b/setup.py
index f5f7afbc..7f2f5103 100644
--- a/setup.py
+++ b/setup.py
@@ -31,6 +31,7 @@
 dependencies = [
     "google-api-core[grpc] >= 1.31.5, <3.0.0dev,!=2.0.*,!=2.1.*,!=2.2.*,!=2.3.0",
     "google-cloud-core >= 1.0.3, < 3.0dev",
+    "protobuf<4.0.0dev",
 ]
 extras = {}