The Cornett or Cornetto is a musical instrument, not to be confused with the cornet.

A cornett is an early wind instrument from the Renaissance period. It is a tube, typically about 3/4 metre (2 feet) long, made of ivory or wood with woodwind-style fingerholes and a small brass instrument-like (lip-vibrated) mouthpiece. Although it may thus seem difficult to know whether to classify it as brass or woodwind, it is undoubtedly brass, and the Hornbostel-Sachs system of musical instrument classification places it alongside other brass instruments such as the trumpet.

Cheaper modern cornetts are made in ABS resin, a plastic also used for making cheaper clarinets and other woodwind instruments (especially recorders).

Purist cornett players tend to use a smaller mouthpiece, whereas those needing to make a compromise--often with the need to go on playing modern brass instruments--may use a much larger mouthpiece, sometimes a trumpet mouthpiece ground down on a lathe so that only the cup and a minimal stub which fits the cornett's mouthpiece receiver are left.

Historically, the cornett was often used in consort with sackbutts (2 cornetts, 3 sackbutts) often to double a church choir. This was particularly popular in Venetian churches, where extensive instrumental accompaniament was encouraged, particularly in use with antiphonal choirs.

The cornett was, like almost all renaissance instruments, made in a complete family, the different sizes being the high cornettino, the cornetto, the tenor cornett (or lizard) and the rare bass cornett (the serpent was preferred to the bass cornett). Other versions include the mute cornett, which is a straight narrow-bore instrument with no mouthpiece, quiet enough to be used in a consort of viols or even recorders.

The cornett was also used as a virtuoso solo instrument, though not much cornett music survives. The use of the instrument had largely died out by 1700. It was last scored for by Gluck, in his opera Orfeo ed Euridice (he suggested the soprano trombone as an alternative). As a point of interest, Gluck was also the last person to score for the recorder, in the same opera.

Despite Marin Mersenne's description of the sound of the cornett "a ray of sunshine piercing the shadows" it was often badly played. Its upper register sounded somewhat like a trumpet or modern cornet, the lower register resembling the sackbutts that often accompanied it, whereas the middle register gave an indistinct wailing sound that was not attractive when played in isolation. The cornett requires a specialized embouchure that is very tiring to play for any length of time. It was inevitable that the finest players of the instrument would turn their attention to the developing oboe.

As a result of the recent early-music renaissance, the cornett has been re-discovered, and as before attracts the finest players. In some pieces (particularly those of early Baroque composers such as Monteverdi and Schutz) the cornett is indespensible in performance, and the music suffers if other instruments substitue them.

To try to avoid confusion between this instrument and the cornet (with one T), the cornett is often referred to by its Italian name, cornetto.