McDonald's first nationally advertised sandwich featuring a fish fillet was the Filet-O-Fish.
The sandwich was created by franchisee Lou Groen in Cincinnati, Ohio, where many customers observed meatless eating on Fridays. Groen proposed a fish sandwich as a way to attract business on days when hamburger sales were weaker. McDonald's tested the product before adding it to the national menu in 1963.
The Filet-O-Fish became an important example of branding shaped by local market conditions. Its name clearly identified the product, while later advertising made the sandwich a permanent part of McDonald's identity. The fish sandwich also helped the chain broaden its menu without abandoning its fast-service format.
The product is sometimes described as McDonald's first non-beef sandwich, but that wording can oversimplify the story. Its major historical significance is that it was the chain's first nationally marketed fish sandwich and one of its earliest enduring menu brands.