The cimbasso (English: /tʃɪmˈbɑːsoʊ/ chim-BAH-soh, Italian: [tʃimˈbaso]) is a low brass instrument that covers the same range as a tuba or contrabass trombone. First appearing in Italy in the early 19th century as an upright serpent, the term cimbasso came to denote several instruments that could play the lowest brass part in 19th century Italian opera orchestras. The modern cimbasso design, first appearing as the trombone basso Verdi in the 1880s, has four to six rotary valves (or occasionally piston valves), a forward-facing bell, and a predominantly cylindrical bore. These features lend its sound to the bass of the trombone family rather than the tuba, and its valves allow for more agility than a contrabass trombone. Like the modern contrabass trombone, it is most often pitched in F, although models are occasionally made in E♭ and low C or B♭.

In the modern orchestra, cimbasso parts are usually played by tuba players as a doubling instrument. Although most commonly used for performances of late Romantic Italian opera, it has since found increased and more diverse use. Jazz musician Mattis Cederberg [sv] uses cimbasso in big bands and as a solo instrument. Cimbasso is now commonly called for in film and video game soundtracks. Los Angeles tuba players Tommy Johnson, Doug Tornquist and Jim Self have featured on many Hollywood recordings playing cimbasso, particularly since the popularisation of loud, low-brass heavy orchestral soundtracks.

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 term was used loosely to refer to the lowest bass instrument available in the brass family, which changed over the course of the 19th century. In the mid-20th century the word cimbasso began to be used in German-speaking countries to refer to slide contrabass trombones in F. This vagueness long impeded research into the instrument's history.

The first uses of a cimbasso in Italian opera scores from the early 19th century referred to a narrow-bore upright serpent similar to the basson russe (lit. 'Russian bassoon'), which were in common use in military bands of the time. These instruments were constructed from wooden sections like a bassoon, with a trombone-like brass bell, sometimes in the shape of a buccin-style dragon's head. Fingering charts published in 1830 indicate these early cimbassi were most likely to have been pitched in C.

Later, the term cimbasso was extended to a range of instruments, including the ophicleide and early valved instruments, such as the Pelittone and other early forms of the more conical bass tuba. As this progressed, the term cimbasso was used to refer to a more blending voice than the "basso tuba" or "bombardone", and began to imply the lowest trombone.

By 1872, Verdi expressed his displeasure about "that devilish bombardone" (referring to an early valved tuba) as the bass of the trombone section for his La Scala première of Aida, preferring a "trombone basso". By the time of his opera Otello in 1887, Milan instrument maker Pelitti [it] had produced the trombone basso Verdi (sometimes trombone contrabbasso Verdi, or simply trombone Verdi). Although no Pelitti instruments, photographs or diagrams survive, it was a contrabass trombone in low 18′ B♭ wrapped in a compact form with 3 or 4 rotary valves. Verdi and Puccini both wrote for this instrument in their later operas, although confusingly, they often referred to it as the trombone basso, to distinguish it from the tenor trombones. This instrument blended well with the usual Italian section of three valve trombones, and was the prototype for the modern cimbasso.

By the early 20th century the tuba was used in Italy for cimbasso parts, and the trombone Verdi, made mainly by Milanese and Bohemian manufacturers, disappeared from Italian orchestras. In 1959 German instrument maker Hans Kunitz developed a slide contrabass trombone in F with two valves based on a 1929 patent by Berlin trombonist Ernst Dehmel. These were built in the 1960s by Gebr. Alexander and named "cimbasso" trombones, and subsequently by other German manufacturers, notably Thein. The modern cimbasso found today emerged in Germany in 1985 by the brass instrument maker Josef Meinl, its design ultimately descended from the Pelitti trombone Verdi design. A contrabass trombone in F but fitted with the valves and fingering of a modern F tuba wrapped in-front of the player, it was quickly adopted throughout Europe by players and other makers, including Bavarian maker Thein Brass.

The modern cimbasso is usually built with four to five rotary valves (or occasionally piston valves), a forward-facing bell, and a cylindrical bore. These features lend its sound to the bass of the trombone family rather than the tuba, and its valves allow for more agility than a contrabass trombone. Like the modern contrabass trombone, it is most often pitched in 12-foot (12′) F, although instruments are made in 13′ E♭ and occasionally low 16′ C or 18′ B♭. A Wiener Kontrabaßposaune model in F with six valves was developed in the late 1990s by the Austrian tubist Gerhard Zechmeister. In order to be familiar with players of the Wiener Konzerttuba, it uses the same six-valve fingering.