The kithara or cithara (Greek: κιθάρα, kithāra, Latin: cithara) was an ancient Greek musical instrument in the lyre or lyra family. In modern Greek the word kithara has come to mean "guitar" (a word whose origins are found in kithara).

The kithara was a professional version of the two-stringed lyre. As opposed to the simpler lyre, which was a folk-instrument, the kithara was primarily used by professional musicians, called kitharodes. The kithara's origins are likely Asiatic. The barbiton was a bass version of the kithara^{[citation needed]} popular in the eastern Aegean and ancient Asia Minor.

The kithara had a deep, wooden sounding box composed of two resonating tables, either flat or slightly arched, connected by ribs or sides of equal width. At the top, its strings were knotted around the crossbar or yoke (zugon) or to rings threaded over the bar, or wound around pegs. The other end of the strings was secured to a tail-piece after passing over a flat bridge, or the tail-piece and bridge were combined. Most vase paintings show kitharas with seven strings, in agreement with ancient authors, but these also mention that occasionally a skillful kitharode would use more than the conventional seven strings.