The Royal Order of the Holy Mackerel

 is a secret society in Gravity Falls, Oregon.

History
Its name is first revealed in the online game Mystery Shack Mystery. It is later 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.

Appearance of the banner
The banner is a yellowish color with a Pharaoh's head painted on it. There are stars above it, and a crest below it.

Trivia

 * "holymackeral" is the name of Alex Hirsch's DeviantArt account, which is currently inactive.
 * The design is very reminiscent of the Shriner's symbol. And whose fez Alex has been seen wearing during Mystery Tour 2013.