American football, referred to simply as football in the United States and Canada and also known as gridiron football,[nb 1] is a team sport played by two teams of eleven players on a rectangular field with goalposts at each end. The offense, the team with possession of the oval-shaped football, attempts to advance down the field by running with the ball or throwing it, while the defense, the team without possession of the ball, aims to stop the offense's advance and to take control of the ball for themselves. The offense must advance the ball at least ten yards in four downs or plays; if they fail, they turn over the football to the defense. If they succeed, they are given a new set of four downs to continue the drive. Points are scored primarily by advancing the ball into the opposing team's end zone for a touchdown or kicking the ball through the opponent's goalposts for a field goal. The team with the most points at the end of the game wins. In the case of a tie after four quarters, the game enters overtime.

American football evolved in the United States, originating from the sports of soccer and rugby. The first American football game was played on November 6, 1869, between two college teams, Rutgers and Princeton, using rules based on the rules of soccer at the time; by 1875, the sport had adopted rules similar to rugby union. A set of rule changes drawn up from 1880 onward by Walter Camp, the Father of American Football, established concepts that would define the sport, among them the snap, the line of scrimmage, eleven-player teams, and the concept of downs. Later rule changes legalized the forward pass, created the neutral zone, and specified the size and shape of the football. The modern sport is closely related to Canadian football, which evolved in parallel with and at the same time as the American game; the two sports are considered the primary variants of gridiron football.

American football is the most popular sport in the United States; the most popular forms of the game are professional and college football, with the other major levels being high school and youth football. Over a million Americans played college or high school football in 2022, and the National Football League (NFL) has one of the highest average attendance of any professional sports league in the world. Its championship game, the Super Bowl, ranks among the most-watched club sporting events globally. Other professional and amateur leagues exist worldwide, but the sport does not have the international popularity of other American sports like baseball or basketball; the sport maintains a growing following in the rest of North America, Europe, Brazil, and Japan. Flag football, a variant of the sport, will be played at the 2028 Summer Olympics.