diff --git a/python/ray/_private/worker.py b/python/ray/_private/worker.py
index d339a9ad29cf..940ff043597c 100644
--- a/python/ray/_private/worker.py
+++ b/python/ray/_private/worker.py
@@ -66,7 +66,7 @@
     ensure_token_if_auth_enabled,
     maybe_enable_token_auth_if_token_available,
 )
-from ray._private.client_mode_hook import client_mode_hook
+from ray._private.client_mode_hook import client_mode_hook, client_mode_should_convert
 from ray._private.function_manager import FunctionActorManager
 from ray._private.inspect_util import is_cython
 from ray._private.ray_logging import (
@@ -111,6 +111,7 @@
 from ray.experimental.tqdm_ray import RAY_TQDM_MAGIC
 from ray.runtime_env.runtime_env import _merge_runtime_env
 from ray.util.annotations import Deprecated, PublicAPI
+from ray.util.client import num_connected_contexts
 from ray.util.debug import log_once
 from ray.util.scheduling_strategies import PlacementGroupSchedulingStrategy
 from ray.util.tracing.tracing_helper import _import_from_string
@@ -1757,6 +1758,9 @@ def sigterm_handler(signum, frame):
         unknown = ", ".join(kwargs)
         raise RuntimeError(f"Unknown keyword argument(s): {unknown}")
 
+    if client_mode_should_convert() and num_connected_contexts() == 0:
+        raise RuntimeError("Ray Client is already initialized in this process.")
+
     # Try to increase the file descriptor limit, which is too low by
     # default for Ray: https://github.com/ray-project/ray/issues/11239
     try:
diff --git a/python/ray/tests/test_client_multi.py b/python/ray/tests/test_client_multi.py
index 69e8b7254691..1b07c66271c0 100644
--- a/python/ray/tests/test_client_multi.py
+++ b/python/ray/tests/test_client_multi.py
@@ -3,6 +3,7 @@
 import pytest
 
 import ray
+from ray._common.network_utils import find_free_port
 
 
 def test_multi_raise_exception_on_non_client(shutdown_only):
@@ -201,6 +202,27 @@ def get(idx):
     assert ret == [0, 1]
 
 
+@pytest.mark.skipif(
+    sys.platform == "win32", reason="PSUtil does not work the same on windows."
+)
+@pytest.mark.parametrize(
+    "call_ray_start",
+    ["ray start --head --ray-client-server-port 25308 --port 0"],
+    indirect=True,
+)
+def test_multi_client_connect_after_client_connect_failed(call_ray_start):
+    @ray.remote
+    def hello():
+        return "world"
+
+    with pytest.raises(ConnectionError):
+        ray.init(f"ray://localhost:{find_free_port()}")
+
+    cli = ray.init("ray://localhost:25308", allow_multiple=True)
+    with cli:
+        assert ray.get(hello.remote()) == "world"
+
+
 if __name__ == "__main__":
     # The following should be removed after
     # https://github.com/ray-project/ray/issues/20355
diff --git a/python/ray/tests/test_ray_init.py b/python/ray/tests/test_ray_init.py
index 0024408c7253..c92d79027002 100644
--- a/python/ray/tests/test_ray_init.py
+++ b/python/ray/tests/test_ray_init.py
@@ -12,8 +12,9 @@
 import pytest
 
 import ray
-from ray._common.network_utils import build_address, parse_address
+from ray._common.network_utils import build_address, find_free_port, parse_address
 from ray._private import ray_constants
+from ray._private.client_mode_hook import disable_client_hook
 from ray._private.test_utils import persistent_gcs_test_enabled
 from ray.client_builder import ClientContext
 from ray.cluster_utils import Cluster
@@ -359,6 +360,32 @@ def test_ray_init_with_runtime_env_as_object(
     assert "gcs://" in parsed_runtime_env["py_modules"][0]
 
 
+@pytest.mark.parametrize(
+    "call_ray_start",
+    ["ray start --head --ray-client-server-port 25307 --port 0"],
+    indirect=True,
+)
+def test_ray_init_after_client_connect_succeeded(call_ray_start):
+    ray.init("ray://localhost:25307")
+
+    try:
+        # A local ray.init() while the Ray Client connection is open must not raise.
+        ray.init()
+    finally:
+        # While client mode is on, ray.shutdown() is routed to the Ray Client.
+        # Bypass the client hook so ray.shutdown() disconnects the local driver instead.
+        with disable_client_hook():
+            ray.shutdown()
+
+
+def test_ray_init_after_client_connect_failed(shutdown_only):
+    with pytest.raises(ConnectionError):
+        ray.init(f"ray://localhost:{find_free_port()}")
+
+    with pytest.raises(RuntimeError, match="Ray Client is already initialized"):
+        ray.init()
+
+
 def test_shutdown_wait_for_processes(shutdown_only):
     ray.init()
     node = ray._private.worker._global_node