A fullback (FB) is a position in the offensive backfield in gridiron football, and is one of the two running back positions along with the halfback. Fullbacks are typically larger than halfbacks and in most offensive schemes the fullback's duties are split among power running, pass catching, and blocking for both the quarterback and the other running back.

Many great runners in the history of American football have been fullbacks, including Jim Brown, Marion Motley, Bronko Nagurski, Jim Taylor, Franco Harris, Larry Csonka, John Riggins, Christian Okoye, and Levi Jackson. However, many of these runners would retroactively be labeled as halfbacks, due to their position as the primary ball carrier; they were primarily listed as fullbacks due to their size and did not often perform the run-blocking duties expected of modern fullbacks. Examples of players who have excelled at the hybrid running–blocking–pass-catching role include Vonta Leach, Mike Alstott, William Henderson, Daryl Johnston, Kyle Juszczyk, John Kuhn, Le'Ron McClain, Lorenzo Neal, Marcel Reece, and Tom Rathman.