diff --git a/gateway/platforms/mattermost.py b/gateway/platforms/mattermost.py
index 574f465b1dd..6bfa6ac4372 100644
--- a/gateway/platforms/mattermost.py
+++ b/gateway/platforms/mattermost.py
@@ -249,6 +249,23 @@ class MattermostAdapter(BasePlatformAdapter):
 
         logger.info("Mattermost: disconnected")
 
+
+    async def _resolve_root_id(self, post_id: str) -> str:
+        """Resolve a post_id to the thread root_id for Mattermost.
+
+        Mattermost requires root_id to be the *root* post of a thread.
+        If the post is a reply (has its own root_id), we must use that
+        root_id instead.  Using a reply's own ID as root_id causes
+        "Invalid RootId parameter" errors.
+        """
+        if not post_id:
+            return post_id
+        # Check if this post has a root_id (meaning it's a reply)
+        data = await self._api_get(f"posts/{post_id}")
+        if data and data.get("root_id"):
+            return data["root_id"]
+        return post_id
+
     async def send(
         self,
         chat_id: str,
@@ -271,7 +288,10 @@ class MattermostAdapter(BasePlatformAdapter):
             }
             # Thread support: reply_to is the root post ID.
             if reply_to and self._reply_mode == "thread":
-                payload["root_id"] = reply_to
+                # Ensure root_id points to the thread root, not a reply.
+                # Mattermost rejects non-root post IDs as root_id.
+                resolved_root = await self._resolve_root_id(reply_to)
+                payload["root_id"] = resolved_root
 
             data = await self._api_post("posts", payload)
             if not data or "id" not in data:
@@ -451,7 +471,7 @@ class MattermostAdapter(BasePlatformAdapter):
             "file_ids": [file_id],
         }
         if reply_to and self._reply_mode == "thread":
-            payload["root_id"] = reply_to
+            payload["root_id"] = await self._resolve_root_id(reply_to)
 
         data = await self._api_post("posts", payload)
         if not data or "id" not in data:
@@ -490,7 +510,7 @@ class MattermostAdapter(BasePlatformAdapter):
             "file_ids": [file_id],
         }
         if reply_to and self._reply_mode == "thread":
-            payload["root_id"] = reply_to
+            payload["root_id"] = await self._resolve_root_id(reply_to)
 
         data = await self._api_post("posts", payload)
         if not data or "id" not in data:
diff --git a/gateway/run.py b/gateway/run.py
index 8a25c7a441b..9512060d7cb 100644
--- a/gateway/run.py
+++ b/gateway/run.py
@@ -15063,7 +15063,7 @@ class GatewayRunner:
         ) if _progress_thread_id else None
         _progress_reply_to = (
             event_message_id
-            if source.platform == Platform.FEISHU and source.thread_id and event_message_id
+            if source.platform in (Platform.FEISHU, Platform.MATTERMOST) and source.thread_id and event_message_id
             else None
         )