A halfback (HB) is an offensive position in American football, whose duties involve lining up in the offensive backfield and carrying the ball on most rushing plays, i.e. a running back. When the principal ball carrier lines up deep in the backfield, and especially when that player is placed behind another player (usually a blocking back), as in the I formation, that player is instead referred to as a tailback.

Sometimes the halfback can catch the ball from the backfield on short passing plays as they are an eligible receiver. Occasionally, they line up as additional wide receivers. When not running or catching the ball, the primary responsibility of a halfback is to aid the offensive linemen in blocking, either to protect the quarterback or another player carrying the football.