diff --git a/integration_tests/samples/socket_mode/builtin_proxy_auth_example.py b/integration_tests/samples/socket_mode/builtin_proxy_auth_example.py
index 0254121e5..be65ddba2 100644
--- a/integration_tests/samples/socket_mode/builtin_proxy_auth_example.py
+++ b/integration_tests/samples/socket_mode/builtin_proxy_auth_example.py
@@ -14,8 +14,8 @@
 from slack_sdk.socket_mode import SocketModeClient
 
 # https://github.com/seratch/my-proxy-server
-# go build && ./my-proxy-sever -a
-proxy_url = "http://user:pass@localhost:9000"
+# go build && my-proxy-server -a -u user -p pass/word
+proxy_url = "http://user:pass%2Fword@localhost:9000"
 
 client = SocketModeClient(
     app_token=os.environ.get("SLACK_SDK_TEST_SOCKET_MODE_APP_TOKEN"),
diff --git a/slack_sdk/socket_mode/builtin/internals.py b/slack_sdk/socket_mode/builtin/internals.py
index 25c69360b..44b4a9a08 100644
--- a/slack_sdk/socket_mode/builtin/internals.py
+++ b/slack_sdk/socket_mode/builtin/internals.py
@@ -12,7 +12,7 @@
 from logging import Logger
 from threading import Lock
 from typing import Tuple, Optional, Union, List, Callable, Dict
-from urllib.parse import urlparse
+from urllib.parse import urlparse, unquote
 
 from .frame_header import FrameHeader
 
@@ -59,7 +59,9 @@ def _establish_new_socket_connection(
         message = [f"CONNECT {server_hostname}:{server_port} HTTP/1.0"]
         if parsed_proxy.username is not None and parsed_proxy.password is not None:
             # In the case where the proxy is "http://{username}:{password}@{hostname}:{port}"
-            raw_value = f"{parsed_proxy.username}:{parsed_proxy.password}"
+            raw_value = (
+                f"{unquote(parsed_proxy.username)}:{unquote(parsed_proxy.password)}"
+            )
             auth = b64encode(raw_value.encode("utf-8")).decode("ascii")
             message.append(f"Proxy-Authorization: Basic {auth}")
         if proxy_headers is not None: