A crossbow is a ranged weapon using an elastic launching device consisting of a bow-like assembly called a prod, mounted horizontally on a main frame called a tiller, which is hand-held in a similar fashion to the stock of a long firearm. Crossbows shoot arrow-like projectiles called bolts or quarrels. A person who shoots crossbow is called a crossbowman or an arbalist (after the arbalest, a European crossbow variant used during the 12th century).

Crossbows and bows use the same launch principle, but an archer must maintain a bow's draw by pitching the bowstring with fingers, pulling it back with arm and back muscles and then holding that form in order to aim. This demands significant physical strength. A crossbow has a locking mechanism to maintain the draw, limiting the shooter's exertion to pulling the string into the lock and then releasing the shot by depressing a lever/trigger. This enables a crossbowman to handle more draw weight, and to hold it with significantly less physical strain, thus potentially achieving better precision.

The earliest known crossbows were made in the first millennium BC, not later than the 7th century BC in ancient China, and not later than the 1st century AD in Greece (as the gastraphetes); each civilization developed the weapon independently. Crossbows brought about a major shift in the role of projectile weaponry in wars, such as during Qin's unification wars and later the Han campaigns against northern nomads and western states. The medieval European crossbow was called by many names, including "crossbow" itself; most of these names derived from the word ballista, an ancient Greek torsion siege engine similar in appearance but different in design principle.

In modern times, firearms have largely supplanted bows and crossbows as weapons of warfare. However, crossbows still remain widely used for competitive shooting sports and hunting, or for relatively silent shooting.