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.

The offensive backfield is an ambiguous generic term of football, which includes: (1) a place, namely the area of an American football field behind the line of scrimmage; and (2) a group classification of certain players positioned there, i.e., members of offense who begin plays behind the line, typically including any backs on the field, such as the quarterback, halfbacks and fullback. There are rules specific to the manner of play in that area.