In the sport of association football, a defender is an outfield player whose primary role is to prevent the opposition from attacking.

There are four types of defender: centre-back, sweeper, full-back and wing-back. The centre-back and full-back positions are essential in most modern formations. The sweeper and wing-back roles are more specialised for certain formations.

The job of the centre-back (also known as the centre-half, central defender, or stopper) is to stop opposing players, particularly the strikers, from scoring, and to bring the ball out from their penalty area. As their name suggests, they play in a central position.

In the modern game, most teams employ two centre-backs, stationed in front of the goalkeeper. There are two main defensive strategies used by centre-backs: the zonal defence, where each centre-back covers a specific area of the pitch; and man-to-man marking, where each centre-back has the job of covering a particular opposition player.

The sweeper is a more versatile type of centre-back who "sweeps up" the ball if an opponent manages to breach the defensive line. His position is rather more fluid than other defenders who man-mark their designated opponents. Because of this, the position is sometimes referred to as libero ([ˈlibero]; from the Italian word meaning "free", as used by the sports journalist Gianni Brera). Though the sweeper may be expected to build counter-attacking moves, and as such requires better ball control and passing ability than a typical centre-back, his talents are often confined to the defensive realm. For example, the catenaccio system of play, used in Italian football in the 1960s, employed a purely defensive sweeper who only "roamed" around the back line.