Filet-O-Fish was introduced in 1963 as McDonald's first nationally marketed sandwich besides hamburgers.
Franchisee Lou Groen created the sandwich in Cincinnati, where many customers observed meatless eating on Fridays. The product used a breaded fish fillet, cheese, and tartar sauce on a bun, giving McDonald's a branded menu option for customers seeking an alternative to beef.
The Filet-O-Fish became an important example of local market needs influencing national branding. Its name clearly identified the product while preserving the short, memorable naming style used by McDonald's menu items.
The sandwich is sometimes confused with the later Big Mac, which was introduced in 1967, or the Quarter Pounder, which arrived in 1971. The Filet-O-Fish predates both and helped broaden McDonald's menu identity.