diff --git a/setup.py b/setup.py
index 86b34924..c6f059c9 100644
--- a/setup.py
+++ b/setup.py
@@ -33,7 +33,8 @@
     # https://github.com/googleapis/google-cloud-python/issues/10566
     "google-api-core[grpc] >= 1.31.5, <3.0.0dev,!=2.0.*,!=2.1.*,!=2.2.*,!=2.3.0",
     "grpc-google-iam-v1 >= 0.12.4, <1.0.0dev",
-    "proto-plus >= 1.15.0",
+    "proto-plus >= 1.15.0, <2.0.0dev",
+    "protobuf >= 3.19.0, <4.0.0dev",
 ]
 
 extras = {"libcst": "libcst >= 0.2.5"}
diff --git a/testing/constraints-3.6.txt b/testing/constraints-3.6.txt
index 71613fcc..5b0f01ea 100644
--- a/testing/constraints-3.6.txt
+++ b/testing/constraints-3.6.txt
@@ -9,3 +9,4 @@ google-api-core==1.31.5
 grpc-google-iam-v1==0.12.4
 libcst==0.2.5
 proto-plus==1.15.0
+protobuf==3.19.0