In association football, the goalkeeper occupies a position that represents the last line of defense between the opponent's offense and his own team's goal. The primary role of the goalkeeper is to defend his team's goal and prevent the opposition from scoring a goal. The goalkeeper is the only player who is permitted to touch the ball with his hands or arms in open play. Each team is required to have a goalkeeper on the field at all times during a match. If a goalkeeper is forced to leave the field due to injury or being sent off, another player must occupy the post, even if the team has no substitute goalkeeper available and/or has used up its allotted substitutions.

Goalkeeper is often abbreviated GK in lineup cards, match reports, and TV captions. The terms keeper and goalie are also commonly used. This position is colloquially referred as 'custodian' or 'the man between the posts'.

When assigning numbers to players on the team, if a squad number system is not in use, the number 1 shirt is usually reserved for the goalkeeper. Notable exceptions include Ubaldo Fillol, who wore the numbers 5 and 7 at the 1978 and 1982 FIFA World Cups, and Vítor Baía, the Portuguese keeper who wore 99 in the latter part of his career.