In baseball, a foul ball is a batted ball that:

• Settles on foul territory between home and first base or between home and third base, or
• Bounces and then goes past first or third base on or over foul territory, or
• Has its first bounce occur in foul territory beyond first or third base, or
• While on or over foul territory, touches the person of an umpire or player, or any object foreign to the natural ground. By interpretation, a batted ball that touches a batter while in his batter's box is foul regardless of whether it is over foul territory.

The entirety of the batted ball must be on or over foul territory in order to be adjudged foul in the above situations; otherwise it is a fair ball that forces the batter to attempt to reach first base.

A foul fly shall be judged according to the relative position of the ball and the foul line, including the foul pole, and not as to whether the fielder is on foul or fair territory at the time he touches the ball. If the foul ball gets caught, then it would be judged as an out.

Additionally, ballpark ground rules may specify that batted balls striking certain fixed objects such as railings, nets, or a roof if present are foul balls.

Foul territory or foul ground is defined as that part of the playing field outside the first and third base lines extended to the fence and perpendicularly upwards. Note: the foul lines and foul poles are not part of foul territory.

In general, when a batted ball is ruled a foul ball, the ball is dead, all runners must return to their time-of-pitch base without liability to be put out, and the batter returns to home plate to continue his turn at bat. A strike is issued for the batter if he had fewer than two strikes. If the batter already has two strikes against him when he hits a foul ball, a strike is not issued unless the ball was bunted to become a foul ball, in which case a third strike is issued and a strikeout recorded for the batter and pitcher. A strike is, however, recorded for the pitcher for every foul ball the batter hits, regardless of the count. If any member of the fielding team catches a foul ball before it touches the ground or lands outside the field perimeter, the batter is out. However, the caught ball is in play and base runners may attempt to advance.

A foul ball is different from a foul tip, in which the ball makes contact with the bat, travels directly to the catcher's hands, and is caught. In this case, the ball remains live and a strike is added to the batter's count. A batter who hits a foul tip with two strikes on the count is out.

In kickball, foul ball does not make a strike, but four foul balls make an out.