In association football, a substitute is a player who is brought on to the pitch during a match in exchange for an existing player. Substitutions are generally made to replace a player who has become tired or injured, or who is performing poorly, or for tactical reasons (such as bringing a striker on in place of a defender). A player who has been substituted during a match takes no further part in the game, in games played under the standard International Football Association Board Laws of the Game.

Substitutions were officially added to the Laws of the Game in 1958. Prior to this most games were played with no changes permitted at all, with occasional exceptions in cases of extreme injury or players not arriving to matches on time.

The number of substitutes has risen over time as well as the number of reserve players allowed to be nominated. It is now common for games to allow a maximum of 5 substitutions; some competitions allow for an additional substitution when playing extra time. A maximum of 3 "substitution opportunities" are provided to a side during normal time, and an extra opportunity during extra time. Substitutions can be made during half-time breaks during normal and extra time, and full time breaks (before the start of extra time), but do not count as substitution opportunities. There is also a provision of an additional substitution beyond whatever limits the match is being played under to be used specifically for a player who has sustained a concussion.

Teams choose a substitute player from a pre-selected set of reserve players, these players typically sit in the technical area with the coaches, and are said to be "on the bench". When the substitute enters the field of play it is said they have come on or have been brought on, while the player they are substituting for is coming off, or being brought off or substituted. This pool of reserve players has also steadily increased in most competitions where they now allow 5, 7 or 9 reserves while in international competitive tournaments it is common that every player selected in the tournament squad (usually 23 players total) is an eligible substitute if they aren't suspended from the game.

A player who is noted for frequently making appearances, or scoring important goals, as a substitute is often informally known as a "super sub".

The origin of football substitutes goes back to at least the early 1860s as part of English public school football games. The original use of the term substitute in football was to describe the replacement of players who failed to turn up for matches. For example, in 1863, a match reports states: "The Charterhouse eleven played a match in cloisters against some old Carthusians but in consequence of the non-appearance of some of those who were expected it was necessary to provide three substitutes." The substitution of absent players happened as early as the 1850s, for example from Eton College where the term emergencies is used. Numerous references to players acting as a "substitute" occur in matches in the mid-1860s where it is not indicated whether these were replacements of absent players or of players injured during the match.

While substitution during games was first added to the Laws of the Game in 1958, there are recorded instances of substitution being permitted on earlier occasions. A friendly in Glasgow between Lancelot and Crosshill in November 1875 saw a Lancelot player injured and Crosshill "allow[ed] them to put a fresh man in his place". The earliest known use of a substitute in a competitive match came on 7 November 1885, when the Lockwood Brothers club used a substitute in an FA Cup first round replay against Notts Rangers, after midfielder F. Brears suffered a broken leg.

The first use of a substitute in international football was on 15 April 1889, in the match between Wales and Scotland at Wrexham. Wales's original goalkeeper, Jim Trainer, failed to arrive; local amateur player Alf Pugh started the match and played for some 20 minutes until the arrival of Sam Gillam, who took over from him. In 1940, in a match between Mandatory Palestine and Lebanon, Mandatory Palestine centre-half Zvi Fuchs was replaced at half-time by Lonia Dvorin following an injury. Also during the qualifying phase for the 1954 World Cup, Horst Eckel of Germany is recorded as having been replaced by Richard Gottinger in their match with the Saarland on 11 October 1953. The use of substitutes in World Cup matches was not allowed until the 1970 tournament.