The Filet-O-Fish was McDonald's first nationally advertised fish sandwich, introduced in 1963.
Franchisee Lou Groen developed the sandwich in Cincinnati, where many customers observed Catholic dietary restrictions on Fridays. The fish sandwich gave McDonald's a meatless option during a period when Friday demand could be difficult for some locations. Corporate testing compared it with a meatless ham sandwich before the fish product was selected.
The Filet-O-Fish became a permanent part of McDonald's branding and menu identity. Its basic presentation has included a breaded fish fillet, a steamed bun, and tartar sauce, although cheese and fish sourcing have varied by market and over time.
The sandwich is sometimes incorrectly dated to the Big Mac era or treated as a later seasonal product. Its national introduction predates the Big Mac's 1968 nationwide launch and helped establish McDonald's as a menu brand with options beyond hamburgers.