The McDonald’s sandwich named after its fish filling is the Filet-O-Fish.
The Filet-O-Fish was created by franchisee Lou Groen, who operated a McDonald’s restaurant in Cincinnati, Ohio. He developed the item to attract Catholic customers who traditionally avoided meat on Fridays, especially during Lent. McDonald’s introduced it in 1963.
Its name is a direct product description: “filet” identifies the portion of fish, while “fish” states the filling plainly. The “O” gives the name a distinctive McDonald’s-style rhythm and helped it stand out from a generic fish sandwich.
The product is sometimes confused with later chicken and seafood items because McDonald’s menus differ around the world. The Filet-O-Fish, however, is one of the chain’s long-running branded menu items and remains closely associated with the company’s historical expansion beyond hamburgers.