The three seconds rule (also referred to as the three-second rule or three in the key, often termed as lane violation) requires that in basketball, a player shall not remain in their opponent’s foul lane for more than three consecutive seconds while that player's team is in control of a live ball in the frontcourt and the game clock is running.

According to the NBA rulebook (the rule is the same for college), once an offensive player brings the ball across half-court, any defensive player inside the 16-foot lane (from the baseline to the free-throw line, commonly referred to “painted area”) must be “actively guarding an opponent within three seconds.” “Actively guarding” means being positioned within arm’s length of an offensive player and in a guarding position.

The NBA implemented the rule during the 2001 season. It states that defensive players cannot stay in the restricted area (the paint or the lane) for more than 3 seconds if they are not actively guarding an offensive player. If a team commits this violation, they are awarded a technical foul.

An offensive player is only allowed to be in the lane for 3 seconds when their team is in possession of the basketball in the front court. The count begins once one foot enters the paint and does not reset until every part of the body leaves the lane or there is a shot attempt.