diff --git a/setup.py b/setup.py
index 6728184..9964fbe 100644
--- a/setup.py
+++ b/setup.py
@@ -29,7 +29,8 @@
     # Until this issue is closed
     # 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",
-    "proto-plus >= 1.15.0",
+    "proto-plus >= 1.15.0, <2.0.0dev",
+    "protobuf >= 3.19.0, <4.0.0dev",
 ]
 
 package_root = os.path.abspath(os.path.dirname(__file__))
diff --git a/testing/constraints-3.6.txt b/testing/constraints-3.6.txt
index be5a64f..786e637 100644
--- a/testing/constraints-3.6.txt
+++ b/testing/constraints-3.6.txt
@@ -6,3 +6,4 @@
 # Then this file should have google-cloud-foo==1.14.0
 google-api-core==1.31.5
 proto-plus==1.15.0
+protobuf==3.19.0