diff --git a/python/ray/tests/test_client_init.py b/python/ray/tests/test_client_init.py
index 9528f1d20..6b6ce8a42 100644
--- a/python/ray/tests/test_client_init.py
+++ b/python/ray/tests/test_client_init.py
@@ -8,7 +8,7 @@ import sys
 import ray.util.client.server.server as ray_client_server
 import ray.core.generated.ray_client_pb2 as ray_client_pb2
 
-from ray.util.client import RayAPIStub
+from ray.util.client import RayAPIStub, CURRENT_PROTOCOL_VERSION
 
 import ray
 
@@ -109,6 +109,45 @@ def test_python_version():
                 python_version="2.7.12",
                 ray_version="",
                 ray_commit="",
+                protocol_version=CURRENT_PROTOCOL_VERSION,
+            )
+
+        # inject mock connection function
+        server_handle.data_servicer._build_connection_response = \
+            mock_connection_response
+
+        ray = RayAPIStub()
+        with pytest.raises(RuntimeError):
+            _ = ray.connect("localhost:50051")
+
+        ray = RayAPIStub()
+        info3 = ray.connect("localhost:50051", ignore_version=True)
+        assert info3["num_clients"] == 1, info3
+        ray.disconnect()
+    finally:
+        ray_client_server.shutdown_with_server(server_handle.grpc_server)
+        time.sleep(2)
+
+
+def test_protocol_version():
+
+    server_handle, _ = ray_client_server.init_and_serve("localhost:50051")
+    try:
+        ray = RayAPIStub()
+        info1 = ray.connect("localhost:50051")
+        local_py_version = ".".join(
+            [str(x) for x in list(sys.version_info)[:3]])
+        assert info1["protocol_version"] == CURRENT_PROTOCOL_VERSION, info1
+        ray.disconnect()
+        time.sleep(1)
+
+        def mock_connection_response():
+            return ray_client_pb2.ConnectionInfoResponse(
+                num_clients=1,
+                python_version=local_py_version,
+                ray_version="",
+                ray_commit="",
+                protocol_version="2050-01-01",  # from the future
             )
 
         # inject mock connection function
diff --git a/python/ray/util/client/__init__.py b/python/ray/util/client/__init__.py
index 9a2d14877..3fdcd4f88 100644
--- a/python/ray/util/client/__init__.py
+++ b/python/ray/util/client/__init__.py
@@ -5,6 +5,10 @@ import logging
 
 logger = logging.getLogger(__name__)
 
+# This version string is incremented to indicate breaking changes in the
+# protocol that require upgrading the client version.
+CURRENT_PROTOCOL_VERSION = "2020-02-01"
+
 
 class RayAPIStub:
     """This class stands in as the replacement API for the `import ray` module.
@@ -35,6 +39,9 @@ class RayAPIStub:
             conn_str: Connection string, in the form "[host]:port"
             secure: Whether to use a TLS secured gRPC channel
             metadata: gRPC metadata to send on connect
+            connection_retries: number of connection attempts to make
+            ignore_version: whether to ignore Python or Ray version mismatches.
+                This should only be used for debugging purposes.
 
         Returns:
             Dictionary of connection info, e.g., {"num_clients": 1}.
@@ -66,7 +73,8 @@ class RayAPIStub:
             self.disconnect()
             raise
 
-    def _check_versions(self, conn_info, ignore_version: bool) -> None:
+    def _check_versions(self, conn_info: Dict[str, Any],
+                        ignore_version: bool) -> None:
         local_major_minor = f"{sys.version_info[0]}.{sys.version_info[1]}"
         if not conn_info["python_version"].startswith(local_major_minor):
             version_str = f"{local_major_minor}.{sys.version_info[2]}"
@@ -77,6 +85,14 @@ class RayAPIStub:
                 logger.warning(msg)
             else:
                 raise RuntimeError(msg)
+        if CURRENT_PROTOCOL_VERSION < conn_info["protocol_version"]:
+            msg = "Client Ray installation out of date:" + \
+                  f" client is {CURRENT_PROTOCOL_VERSION}," + \
+                  f" server is {conn_info['protocol_version']}"
+            if ignore_version:
+                logger.warning(msg)
+            else:
+                raise RuntimeError(msg)
 
     def disconnect(self):
         """Disconnect the Ray Client.
diff --git a/python/ray/util/client/server/dataservicer.py b/python/ray/util/client/server/dataservicer.py
index 709147820..82ddc85c6 100644
--- a/python/ray/util/client/server/dataservicer.py
+++ b/python/ray/util/client/server/dataservicer.py
@@ -8,16 +8,13 @@ from threading import Lock
 
 import ray.core.generated.ray_client_pb2 as ray_client_pb2
 import ray.core.generated.ray_client_pb2_grpc as ray_client_pb2_grpc
+from ray.util.client import CURRENT_PROTOCOL_VERSION
 
 if TYPE_CHECKING:
     from ray.util.client.server.server import RayletServicer
 
 logger = logging.getLogger(__name__)
 
-# This version string is incremented to indicate breaking changes in the
-# protocol that require upgrading the client version.
-CURRENT_PROTOCOL_VERSION = "2020-02-01"
-
 
 class DataServicer(ray_client_pb2_grpc.RayletDataStreamerServicer):
     def __init__(self, basic_service: "RayletServicer"):