Which year did the AFL and NFL officially merge?

The AFL and NFL officially merged in 1970. The merger ended a decade of competition between the established National Football League and the upstart American Football League, creating the modern NFL structure.

The leagues agreed to merge in 1966, but they continued operating separately for four seasons. During that transition, their champions met in the AFL-NFL World Championship Game, later known as the Super Bowl. The merger became operational for the 1970 season, when the combined league was reorganized into the American Football Conference and National Football Conference.

The 26-team merged NFL included all 10 AFL clubs and 16 NFL clubs. The Baltimore Colts, Cleveland Browns, and Pittsburgh Steelers moved from the old NFL into the AFC, joining the former AFL teams; the remaining NFL clubs formed the NFC.

A common mix-up is answering 1966, the year the merger agreement was announced and approved. That agreement set the plan in motion, but 1970 is the canonical answer for when the two leagues officially became one league.