The tuba (UK: /ˈtjuːbə/; US: /ˈtuːbə/) is the largest and lowest-pitched musical instrument in the brass family. As with all brass instruments, the sound is produced by lip vibration – a buzz – into a mouthpiece. It first appeared in the mid-19th century, making it one of the newer instruments in the modern orchestra and concert band, and largely replaced the ophicleide. Tuba is Latin for "trumpet".

A person who plays the tuba is called a tubaist, a tubist, or simply a tuba player. In a British brass band or military band, they are known as bass players.

Prussian Patent No. 19 was granted on 12 September 1835 to the military bandmaster Wilhelm Friedrich Wieprecht and Johann Gottfried Moritz, a Berlin-based instrument maker, for a Baß-Tuba (lit. 'bass tuba') in F. The original Wieprecht and Moritz instrument used five Berlinerpumpen valves, forerunners of the modern Périnet piston valves. The first tenor tuba was invented in 1838 by Moritz's son, Carl Wilhelm Moritz.

The addition of valves made it possible to play low in the harmonic series of the instrument and still have a complete selection of notes. Prior to the invention of valves, brass instruments were limited to notes in the harmonic series, and were thus generally played very high with respect to their fundamental pitch. Harmonics starting three octaves above the fundamental pitch are about a whole step apart, making a useful variety of notes possible.

The ophicleide used a bowl-shaped brass instrument mouthpiece but had keys and tone holes similar to those of a modern saxophone. Another forerunner to the tuba, the serpent, was a bass instrument shaped in a wavy form to make the tone holes accessible to the player. Tone holes change the pitch by providing an intentional leak in the bugle of the instrument, but this system has a pronounced effect on the timbre. By using valves instead, the tuba could produce a smoother tone, which led to its popularity. Tubas were mostly used by French composers, especially Hector Berlioz, who famously used the ophicleide in his compositions Symphonie fantastique and Benvenuto Cellini. These pieces are now normally performed on F or CC tuba.

Adolphe Sax, like Wieprecht, was interested in marketing families of instruments ranging from soprano to bass, and developed a series of brass instruments known as saxhorns. The instruments developed by Sax were generally pitched in E♭ and B♭, while the Wieprecht "basstuba" and the subsequent Červený contrabass tuba were pitched in F and C (see below on pitch systems). Sax's instruments gained dominance in France, and later in Britain and America, as a result of the movements of popular instrument makers such as Gustave Auguste Besson (who moved from France to Britain) and Henry Distin (who eventually found his way to America).

The cimbasso is also seen instead of a tuba in the orchestral repertoire. The Italian word cimbasso, first appearing in the early 19th century, is thought to be a contraction used by musicians of the term corno basso or corno di basso (lit. 'bass horn'), sometimes appearing in scores as c. basso or c. in basso. The original design was inspired by the ophicleide and the bassoon. The cimbasso is sometimes used in historically accurate performances and is commonly called for in film and video game soundtracks.

An orchestra usually has a single tuba, though an additional tuba may be requested. It serves as the bass of the orchestral brass section and it can reinforce the bass voices of the strings and woodwinds. It provides the bass of brass quintets and choirs (though many small brass ensembles will use the euphonium or bass trombone as the lowest voice). It is the principal bass instrument in concert bands, brass bands and military bands, and those ensembles generally have two to four tubas. It is also a solo instrument.