diff --git a/slacker/__init__.py b/slacker/__init__.py
index 50a47ac..2128c88 100644
--- a/slacker/__init__.py
+++ b/slacker/__init__.py
@@ -268,7 +268,7 @@ class Chat(BaseAPI):
     def post_message(self, channel, text=None, username=None, as_user=None,
                      parse=None, link_names=None, attachments=None,
                      unfurl_links=None, unfurl_media=None, icon_url=None,
-                     icon_emoji=None):
+                     icon_emoji=None, thread_ts=None):
 
         # Ensure attachments are json encoded
         if attachments:
@@ -287,7 +287,8 @@ def post_message(self, channel, text=None, username=None, as_user=None,
                              'unfurl_links': unfurl_links,
                              'unfurl_media': unfurl_media,
                              'icon_url': icon_url,
-                             'icon_emoji': icon_emoji
+                             'icon_emoji': icon_emoji,
+                             'thread_ts': thread_ts
                          })
 
     def me_message(self, channel, text):