In association football (soccer), a red card is shown by a referee to signify that a player has been sent off following a serious offence.

A player who has been sent off is required to leave the field of play immediately and must take no further part in the game. The player who has been sent off cannot be replaced during the game; their team must continue the game with one player less.

Law XII of the Laws of the Game (which are set by the International Football Association Board and used by the sport's international governing body FIFA) list the categories of offences for which a player may be sent off. Broadly these are:

1. Serious foul play. This includes a tackle that endangers the safety of an opponent. Some examples include a dangerous sliding tackle from behind, or a two-footed lunging tackle.
2. Violent conduct
3. Spitting at anyone
4. A deliberate hand ball to deny an obvious goal-scoring opportunity by any player other than a goalkeeper in his own penalty area
5. Committing an offence that denies an opponent an obvious goal-scoring opportunity (informally known as a professional foul)
6. Using offensive, insulting or abusive language or gestures
7. Receiving a second caution (yellow card) in one game

In addition to being excluded from participating in the rest of the game, a player who has been sent off may be subject to off-field sanctions. These are usually in the form of suspensions from a playing for a number of future games. The exact punishments are determined by tournament or competition rules (not by the Laws of the Game). However, FIFA in particular has been adamant that a red card in any soccer competition must result in the guilty player being suspended for at least the next game without the right to appeal.

Other sports also use a red card to indicate a permanent expulsion from a game, including handball, rugby union, British rugby league and field hockey.