The Royal Order of the Holy Mackerel

 is a secret society in Gravity Falls, Oregon.

History
It's name is first revealed in the game Mystery Shack Mystery. It is mentioned in Rumble's Revenge, in a cryptogram that says, "There is a secret society in Gravity Falls." Stan has a banner with its symbol in the living room, and in the gift shop of the Mystery Shack.

Trivia

 * Holy Mackerel is the name of one of Alex Hirsh's deviantart accounts.