The cello (/ˈtʃɛloʊ/ CHEL-oh; plural cellos or celli) or violoncello (/ˌvaɪələnˈtʃɛloʊ/ VY-ə-lən-CHEL-oh;Italian pronunciation: [vjolonˈtʃɛllo]) is a bowed string instrument with four strings tuned in perfect fifths. It is a member of the violin family of musical instruments, which also includes the violin and viola.

The cello is used as a solo musical instrument, as well as in chamber music ensembles, string orchestras, as a member of the string section of symphony orchestras, and some rock bands. It is the second-largest bowed string instrument in the modern symphony orchestra, the double bass being the largest.

Cellos were derived from other mid- to large-sized bowed instruments in the 16th century, such as the viola da gamba, and the generally smaller and squarer viola da braccio, and such instruments made by members of the Amati family of luthiers.

Cello parts are generally written in the bass clef, but both tenor and treble clefs are used for higher-range parts.

A person who plays the cello is called a cellist or violoncellist.