The alto trombone is the alto member of the trombone family of brass instruments, smaller than the tenor trombone. It is almost always pitched in E♭ a fourth higher than the tenor, although examples pitched in F are occasionally found. The alto trombone was commonly used from the 16th to the 18th centuries in church music to strengthen the alto voice, particularly in the Mass. Alto trombone parts are usually notated in alto clef.

The trombone is a musical instrument in the brass family. As with all brass instruments, sound is produced when the player's vibrating lips cause the air column inside the instrument to vibrate. Nearly all trombones use a telescoping slide mechanism to alter the pitch instead of the valves used by other brass instruments. The valve trombone is an exception, using three valves similar to those on a trumpet, and the superbone has valves and a slide.