A baritone is a B-flat brass instrument controlled by valves. Sometimes mistaken for a little tuba, it is much smaller, with a smaller bell and higher pitch.

The baritone is very similar to the euphonium, but has cylindrical tubing, is smaller and has a brighter, lighter tone. There are usually three piston valves, although some baritones are made with four valves. The euphonium may have one or two more valves than the baritone.

Structurally, the baritone horn used in a European-style brass band is upright, with the bell pointing upward. The tone of the instrument is midway between the bright sound of the trombone and the mellow timbre of the euphonium. The baritone is almost never used as a solo instrument, as its agility and range are quite limited in comparison with the euphonium. The baritone is generally assigned the tenor part in a four-part setting.

During the 1830s, instrument manufacturers in Germany and Austria began experimenting with designs of brass instruments in the tenor and baritone range, which eventually led to the present-day baritone and euphonium. In 1843, Sommer of Weimar, a German concertmaster, designed a "wide-bore, valved bugle of baritone range," at first called the euphonium in Germany. Its name was later changed to baryton. Ferdinand Hell of Vienna designed a similar instrument to the euphonium, a bass baritone of tenor range, called the hellhorn. Throughout the 19th century, so many similar instruments appeared – the phonikon, saxhorn, kaiserbariton, double-belled euphonium and tenor tuba – in various keys, sizes, shapes and names, that an official conference in instrumentation standardization was convened in 1921 at London's Kneller Hall. Baritones were taken out of wind bands and limited primarily to the brass band.