diff --git a/setup.py b/setup.py
index 9b0178d..3f6e52b 100644
--- a/setup.py
+++ b/setup.py
@@ -50,8 +50,9 @@
         # 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",
-        "grafeas >= 1.1.2",
+        "proto-plus >= 1.15.0, <2.0.0dev",
+        "protobuf >= 3.19.0, <4.0.0dev",
+        "grafeas >= 1.1.2, <2.0dev",
     ),
     python_requires=">=3.6",
     classifiers=[
diff --git a/testing/constraints-3.6.txt b/testing/constraints-3.6.txt
index 71a1281..ca3c4ca 100644
--- a/testing/constraints-3.6.txt
+++ b/testing/constraints-3.6.txt
@@ -7,3 +7,4 @@
 google-api-core==1.31.5
 proto-plus==1.15.0
 grafeas==1.1.2
+protobuf==3.19.0