A euphonium is a medium-sized, tenor-voiced brass instrument that is part of the tuba family and is pitched in Bb exactly an octave higher than the BBb contrabass tuba. It has a conical bore and is often compensating, meaning it has additional tubing to improve intonation. The euphonium is similar to a baritone horn, but the bore size of the baritone horn is typically smaller than that of the euphonium, and the baritone has a primarily cylindrical bore, whereas the euphonium has a predominantly conical bore. The euphonium is named after the Ancient Greek word εὔφωνος euphōnos, meaning "well-sounding" or "sweet-voiced".

The euphonium is an incredibly versatile instrument capable of up to a 5 octave range by professional players, which has the depth of a tuba with the dexterity of a cornet. It is often used in wind ensembles and British-style brass bands. The euphonium functions in an ensemble like the cello in an orchestra.

It is unclear who invented the euphonium, but three people are generally credited with inventing it: Carl Moritz, Ferdinand Sommer, and Adolphe Sax. The euphonium is a relatively recent addition to the repertory of instruments, as it was born out of the invention of the tuba, which was only created a little under 200 years ago.

In general, there are three terms used for euphonium players: euphoniumist, euphophonist, or euphonist. The Conn American-style euphonium is often called a baritone, but it fits very well into the range of measurements of the other traditional-style euphoniums.