The euphonium is the tenor member of the tuba family. It is a valved brass instrument whose bore constantly increases in size from the valve section to the bell - this is called "conical bore", as distinguished from "cylindrical bore" instruments which maintain a constant tubing diameter for a greater portion of the instrument's total length. The euphonium plays in the same range of pitches as the trombone, but its cylindrical bore gives it a more mellow, "rounder" sound compared to the "brassier" sound of a trombone.

Invented by Adolphe Sax, the euphonium is thus a saxhorn.

Other instruments essentially similar to the euphonium include the American-style baritone horn, the British baritone (which is more cylindrical and trombone-like than the American baritone), and the Germany baryton and tenorhorn. All trace their descent to the serpent. A unique American creation was the double-belled euphonium, which featured a second, smaller bell and an extra valve allowing the player to use that bell instead of the main bell. The second bell had a more trombone-like sound.

The euphonium is not often used by symphony orchestras,(its most notable use being its part in Holst's Planets Suite) so its main venues are various wind bands, such as school bands, brass bands, marching bands, and military bands, where it is frequently featured as a solo instrument. The name "euphonium" comes from the Greek for "beautiful-sounding". As the musician Billy Joel wrote, "It's either sadness or euphonium."