In sports, a false start is a disallowed start, usually due to a movement by a participant before (or in some cases after) being signaled or otherwise permitted by the rules to start. Depending on the sport and the event, a false start can result in a penalty against the athlete's or team's field position, a warning that a subsequent false start will result in disqualification, or immediate disqualification of the athlete from further competition.

False starts are common in racing sports (such as swimming, track, sprinting, and motor sports), where differences are made by fractions of a second and where anxiety to get the best start plays a role in the athletes' behavior.

A race that is started without a false start is referred to as a fair start or clean start.

Football games cannot be restarted unless certain conditions are met. For example, both teams need to be in their own half of the field for the start of the game or restarts from goals or half-time and free kicks require players to be a certain distance from the dead ball position. A referee may call the players back if one or more encroach into the wrong part of the field. There is generally no penalty for this type of encroachment, although if the referee considers it to be delaying the restart of the game they can award a yellow card. One famous example was during the 1974 World Cup when Brazil had a free kick near the Zaire penalty area, but the ball was booted far away by Mwepu Ilunga before the restart. At a penalty kick, the restart is blown by the referee before the actual kick takes place. In this situation, "encroachment" may take place, where one or more players from either side go into the penalty area or penalty arc before the kick is done. The goalkeeper can also be called for this offence if one foot leaves the goal line before the kick. A variety of punishments exist depending on which sides were involved and the result of the kick, or the result may stand if one team defends the kick or scores it but their opposition infringed.

In American football and Canadian football, a false start is movement by an offensive player (other than the center) after he has taken a set position. For offensive linemen, this movement might be as minimal as a couple of centimeters, although the rule's intent is to prevent offensive players from unfairly drawing the defense offside. A false start brings a 5-yard penalty. Unlike an offside penalty, where the play is run as usual, the play after a false start penalty immediately becomes dead. This is done to prevent a defensive player reacting to a false start from hitting the quarterback while he is going through the snap count, which would make the quarterback more susceptible to injury.

At the end of the 2005 NFL season, owners complained regarding false start penalties on players whose flinches have little effect upon the start of the play, such as wide receivers. In response, the NFL competition committee has said that they plan to inflict fewer false start penalties on players who line up behind the line of scrimmage.

In the 2023 season the false start penalty was the most issued penalty in the NFL with 618 penalties being issued for 3,026 penalty yards.

In track and field sprints, the sport's governing body, World Athletics, has a rule that if the athlete moves within 0.1 seconds after the gun has fired the athlete has false-started. This figure is based on tests that show the human brain cannot hear and process the information from the start sound in under 0.10 seconds, even though a IAAF-commissioned study indicated in 2009 that top sprinters are able to sometimes react in 0.08 seconds. This rule is only applied at high-level meets where fully automated force or motion sensor devices are built into the starting blocks that are tied via computer with the starter's gun. In the vast majority of lower-level meets, false starts are determined visually by the officials.