diff --git a/smart_open/smart_open_lib.py b/smart_open/smart_open_lib.py
index 9454450e..01c5f12e 100644
--- a/smart_open/smart_open_lib.py
+++ b/smart_open/smart_open_lib.py
@@ -370,7 +370,7 @@ def _open_binary_stream(uri, mode, **kw):
         #
         host = kw.pop('host', None)
         if host is not None:
-            kw['endpoint_url'] = 'http://' + host
+            kw['endpoint_url'] = _add_scheme_to_host(host)
         return smart_open_s3.open(uri.bucket.name, uri.name, mode, **kw), uri.name
     elif hasattr(uri, 'read'):
         # simply pass-through if already a file-like
@@ -395,7 +395,7 @@ def _s3_open_uri(parsed_uri, mode, **kwargs):
     # Get an S3 host. It is required for sigv4 operations.
     host = kwargs.pop('host', None)
     if host is not None:
-        kwargs['endpoint_url'] = 'http://' + host
+        kwargs['endpoint_url'] = _add_scheme_to_host(host)
 
     return smart_open_s3.open(parsed_uri.bucket_id, parsed_uri.key_id, mode, **kwargs)
 
@@ -615,3 +615,8 @@ def _encoding_wrapper(fileobj, mode, encoding=None, errors=DEFAULT_ERRORS):
     else:
         decoder = codecs.getwriter(encoding)
     return decoder(fileobj, errors=errors)
+
+def _add_scheme_to_host(host):
+    if host.startswith('http://') or host.startswith('https://'):
+        return host
+    return 'http://' + host
diff --git a/smart_open/tests/test_smart_open.py b/smart_open/tests/test_smart_open.py
index 461319a7..cc567d2e 100644
--- a/smart_open/tests/test_smart_open.py
+++ b/smart_open/tests/test_smart_open.py
@@ -1163,6 +1163,31 @@ def test_write_text_gzip(self):
             actual = fin.read()
         self.assertEqual(text, actual)
 
+class HostNameTest(unittest.TestCase):
+
+    def test_host_name_with_http(self):
+        host = 'http://a.com/b'
+        expected = 'http://a.com/b'
+        res = smart_open_lib._add_scheme_to_host(host)
+        self.assertEqual(expected, res)
+
+    def test_host_name_without_http(self):
+        host = 'a.com/b'
+        expected = 'http://a.com/b'
+        res = smart_open_lib._add_scheme_to_host(host)
+        self.assertEqual(expected, res)
+
+    def test_host_name_with_https(self):
+        host = 'https://a.com/b'
+        expected = 'https://a.com/b'
+        res = smart_open_lib._add_scheme_to_host(host)
+        self.assertEqual(expected, res)
+
+    def test_host_name_without_http_prefix(self):
+        host = 'httpa.com/b'
+        expected = 'http://httpa.com/b'
+        res = smart_open_lib._add_scheme_to_host(host)
+        self.assertEqual(expected, res)
 
 if __name__ == '__main__':
     logging.basicConfig(format='%(asctime)s : %(levelname)s : %(message)s', level=logging.DEBUG)