An air gun or airgun is a gun that fires projectiles using energy from compressed air or other gases that are mechanically pressurized. This is in contrast to a firearm, which fires projectiles using energy created by burning combustible propellants, i.e. gunpowder.

Air guns come in both long gun (air rifle) and handgun (air pistol) forms. Both types typically propel metallic projectiles that are either diabolo-shaped pellets or spherical shots called BBs, although in recent years Minié ball-shaped cylindro-conoidal projectiles called slugs are gaining more popularity. Certain types of air guns (usually air rifles) may also launch fin-stabilized projectile such as darts (e.g., tranquilizer guns) or hollow-shaft arrows (so-called "airbows").

The first air guns were developed as early as the 16th century, and have since been used in hunting, shooting sport and even in warfare. There are three different power sources for modern air guns, depending on the design: spring-piston, pneumatic, or bottled compressed gas (most commonly carbon dioxide).