diff --git a/telethon/client/chats.py b/telethon/client/chats.py
index b86b4fa5..08242f0e 100644
--- a/telethon/client/chats.py
+++ b/telethon/client/chats.py
@@ -873,6 +873,19 @@ class ChatMethods:
         )
 
         if isinstance(entity, types.InputPeerChannel):
+            # If we try to set these permissions in a megagroup, we
+            # would get a RIGHT_FORBIDDEN. However, it makes sense
+            # that an admin can post messages, so we want to avoid the error
+            if post_messages or edit_messages:
+                # TODO get rid of this once sessions cache this information
+                if entity.channel_id not in self._megagroup_cache:
+                    full_entity = await self.get_entity(entity)
+                    self._megagroup_cache[entity.channel_id] = full_entity.megagroup
+
+                if self._megagroup_cache[entity.channel_id]:
+                    post_messages = None
+                    edit_messages = None
+
             perms = locals()
             return await self(functions.channels.EditAdminRequest(entity, user, types.ChatAdminRights(**{
                 # A permission is its explicit (not-None) value or `is_admin`.
diff --git a/telethon/client/telegrambaseclient.py b/telethon/client/telegrambaseclient.py
index b0882e6c..26f17a92 100644
--- a/telethon/client/telegrambaseclient.py
+++ b/telethon/client/telegrambaseclient.py
@@ -340,6 +340,9 @@ class TelegramBaseClient(abc.ABC):
         self._self_input_peer = None
         self._bot = None
 
+        # A place to store if channels are a megagroup or not (see `edit_admin`)
+        self._megagroup_cache = {}
+
     # endregion
 
     # region Properties