diff --git a/README.rst b/README.rst
index 5f0ef2e..bf88e4c 100644
--- a/README.rst
+++ b/README.rst
@@ -69,6 +69,18 @@ the Threema gateway. Run the following command to see usage information:
 
     $ threema-gateway --help
 
+Setting a different API url
+---------------------------
+
+The default MsgApi URL points to https://msgapi.threema.ch/.
+
+If you are a Threema OnPrem customer or have another reason
+to use a different MsgApi endpoint, you may set an environment variable as follows:
+
+.. code-block:: bash
+
+    $ GATEWAY_API_URL=https://onprem.myinstance.tld/msgapi threema-gateway ...
+
 Examples
 ********
 
diff --git a/tests/test_cli.py b/tests/test_cli.py
index 1300952..dad599f 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -252,7 +252,7 @@ async def test_invalid_id(self, cli):
     @pytest.mark.asyncio
     async def test_insufficient_credits(self, cli):
         with pytest.raises(subprocess.CalledProcessError) as exc_info:
-            id_, secret = pytest.msgapi['msgapi']['nocredit_id'],\
+            id_, secret = pytest.msgapi['msgapi']['nocredit_id'], \
                 pytest.msgapi['msgapi']['secret']
             await cli('send-simple', 'ECHOECHO', id_, secret, input='!')
         assert 'Insufficient credits' in exc_info.value.output
diff --git a/threema/gateway/bin/gateway_client.py b/threema/gateway/bin/gateway_client.py
index 2672a08..7a9f49d 100755
--- a/threema/gateway/bin/gateway_client.py
+++ b/threema/gateway/bin/gateway_client.py
@@ -36,12 +36,24 @@
 
 # Apply mock URL when starting CLI in debug mode
 _test_port = os.environ.get('THREEMA_TEST_API')
+_api_url = os.environ.get('GATEWAY_API_URL')
 if _test_port is not None:
+    if _api_url is not None:
+        raise RuntimeError('GATEWAY_API_URL cannot be set alongside THREEMA_TEST_API')
     _mock_url = 'http://{}:{}'.format('127.0.0.1', _test_port)
     Connection.urls = {key: value.replace('https://msgapi.threema.ch', _mock_url)
                        for key, value in Connection.urls.items()}
     click.echo(('WARNING: Currently running in test mode!'
                 'The Threema Gateway Server will not be contacted!'), err=True)
+else:
+    if _api_url is not None:
+        if not _api_url.startswith('https://'):
+            raise RuntimeError('GATEWAY_API_URL must begin with "https://"')
+        Connection.urls = {key: value.replace(
+            'https://msgapi.threema.ch',
+            _api_url.rstrip('/')
+        )
+            for key, value in Connection.urls.items()}
 
 
 class _MockConnection(AioRunMixin):