diff --git a/tests/slack_sdk/web/test_legacy_web_client_url_format.py b/tests/slack_sdk/web/test_legacy_web_client_url_format.py
new file mode 100644
index 00000000..0ba2f5ae
--- /dev/null
+++ b/tests/slack_sdk/web/test_legacy_web_client_url_format.py
@@ -0,0 +1,35 @@
+from unittest import TestCase
+
+from slack_sdk.web.legacy_client import LegacyWebClient
+from tests.slack_sdk.web.mock_web_api_handler import MockHandler
+from tests.mock_web_api_server import setup_mock_web_api_server, cleanup_mock_web_api_server, assert_received_request_count
+
+
+class TestLegacyWebClientUrlFormat(TestCase):
+    def setUp(self):
+        setup_mock_web_api_server(self, MockHandler)
+        self.client = LegacyWebClient(token="xoxb-api_test", base_url="http://localhost:8888")
+        self.client_base_url_slash = LegacyWebClient(token="xoxb-api_test", base_url="http://localhost:8888/")
+
+    def tearDown(self):
+        cleanup_mock_web_api_server(self)
+
+    def test_base_url_without_slash_api_method_without_slash(self):
+        self.client.api_call("chat.postMessage")
+        assert_received_request_count(self, "/chat.postMessage", 1)
+
+    def test_base_url_without_slash_api_method_with_slash(self):
+        self.client.api_call("/chat.postMessage")
+        assert_received_request_count(self, "/chat.postMessage", 1)
+
+    def test_base_url_with_slash_api_method_without_slash(self):
+        self.client_base_url_slash.api_call("chat.postMessage")
+        assert_received_request_count(self, "/chat.postMessage", 1)
+
+    def test_base_url_with_slash_api_method_with_slash(self):
+        self.client_base_url_slash.api_call("/chat.postMessage")
+        assert_received_request_count(self, "/chat.postMessage", 1)
+
+    def test_base_url_without_slash_api_method_with_slash_and_trailing_slash(self):
+        self.client.api_call("/chat.postMessage/")
+        assert_received_request_count(self, "/chat.postMessage/", 1)
diff --git a/tests/slack_sdk/web/test_web_client_url_format.py b/tests/slack_sdk/web/test_web_client_url_format.py
new file mode 100644
index 00000000..9ef4f932
--- /dev/null
+++ b/tests/slack_sdk/web/test_web_client_url_format.py
@@ -0,0 +1,35 @@
+from unittest import TestCase
+
+from slack_sdk.web import WebClient
+from tests.slack_sdk.web.mock_web_api_handler import MockHandler
+from tests.mock_web_api_server import setup_mock_web_api_server, cleanup_mock_web_api_server, assert_received_request_count
+
+
+class TestWebClientUrlFormat(TestCase):
+    def setUp(self):
+        setup_mock_web_api_server(self, MockHandler)
+        self.client = WebClient(token="xoxb-api_test", base_url="http://localhost:8888")
+        self.client_base_url_slash = WebClient(token="xoxb-api_test", base_url="http://localhost:8888/")
+
+    def tearDown(self):
+        cleanup_mock_web_api_server(self)
+
+    def test_base_url_without_slash_api_method_without_slash(self):
+        self.client.api_call("chat.postMessage")
+        assert_received_request_count(self, "/chat.postMessage", 1)
+
+    def test_base_url_without_slash_api_method_with_slash(self):
+        self.client.api_call("/chat.postMessage")
+        assert_received_request_count(self, "/chat.postMessage", 1)
+
+    def test_base_url_with_slash_api_method_without_slash(self):
+        self.client_base_url_slash.api_call("chat.postMessage")
+        assert_received_request_count(self, "/chat.postMessage", 1)
+
+    def test_base_url_with_slash_api_method_with_slash(self):
+        self.client_base_url_slash.api_call("/chat.postMessage")
+        assert_received_request_count(self, "/chat.postMessage", 1)
+
+    def test_base_url_without_slash_api_method_with_slash_and_trailing_slash(self):
+        self.client.api_call("/chat.postMessage/")
+        assert_received_request_count(self, "/chat.postMessage/", 1)
diff --git a/tests/slack_sdk_async/web/test_web_client_url_format.py b/tests/slack_sdk_async/web/test_web_client_url_format.py
new file mode 100644
index 00000000..3f25dac0
--- /dev/null
+++ b/tests/slack_sdk_async/web/test_web_client_url_format.py
@@ -0,0 +1,45 @@
+import unittest
+
+from slack_sdk.web.async_client import AsyncWebClient
+from tests.slack_sdk_async.helpers import async_test
+from tests.slack_sdk.web.mock_web_api_handler import MockHandler
+from tests.mock_web_api_server import (
+    setup_mock_web_api_server_async,
+    cleanup_mock_web_api_server_async,
+    assert_received_request_count_async,
+)
+
+
+class TestAsyncWebClientUrlFormat(unittest.TestCase):
+    def setUp(self):
+        setup_mock_web_api_server_async(self, MockHandler)
+        self.client = AsyncWebClient(token="xoxb-api_test", base_url="http://localhost:8888")
+        self.client_base_url_slash = AsyncWebClient(token="xoxb-api_test", base_url="http://localhost:8888/")
+
+    def tearDown(self):
+        cleanup_mock_web_api_server_async(self)
+
+    @async_test
+    async def test_base_url_without_slash_api_method_without_slash(self):
+        await self.client.api_call("chat.postMessage")
+        await assert_received_request_count_async(self, "/chat.postMessage", 1)
+
+    @async_test
+    async def test_base_url_without_slash_api_method_with_slash(self):
+        await self.client.api_call("/chat.postMessage")
+        await assert_received_request_count_async(self, "/chat.postMessage", 1)
+
+    @async_test
+    async def test_base_url_with_slash_api_method_without_slash(self):
+        await self.client_base_url_slash.api_call("chat.postMessage")
+        await assert_received_request_count_async(self, "/chat.postMessage", 1)
+
+    @async_test
+    async def test_base_url_with_slash_api_method_with_slash(self):
+        await self.client_base_url_slash.api_call("/chat.postMessage")
+        await assert_received_request_count_async(self, "/chat.postMessage", 1)
+
+    @async_test
+    async def test_base_url_without_slash_api_method_with_slash_and_trailing_slash(self):
+        await self.client.api_call("/chat.postMessage/")
+        await assert_received_request_count_async(self, "/chat.postMessage/", 1)