The barbiton, or barbitos (Gr: βάρβιτον or βάρβιτος; Lat. barbitus), is an ancient stringed instrument related to the lyre known from Greek and Roman classics.

The Greek instrument was a bass version of the kithara, and belonged in the zither family, but in medieval times, the same name was used to refer to the barbat; a different instrument that is a variety of lute.

The barbat or barbud, began being translated into Latin as barbiton sometime during the late Middle Ages, a mistaken practice which has passed into English and other European languages through long misuse. The barbat is an unrelated lute-family instrument developed in Persia, whereas the barbiton (a bass kithara) was developed in Greek-speaking western Anatolia, where it was popular, and spread into the rest of the Aegean.

Since neither instrument was familiar to European musicians of the late Middle Ages – both had fallen out of use in the occident sometime between the mid-Imperial period and the end of the Roman empire – the error was neither caught nor corrected. The mistake seems to be perpetually dredged up from the earlier erroneous texts.

The barbiton (bass kithara) was rare and considered exotic in the Hellenic world – only popular in Anatolia and the eastern Aegean. There are much fewer descriptions of it than the well-regarded kithara, and some depictions of it on painted vases were made by painters and sculptors who may have rarely seen one themselves, if ever. Consequently, ethno-musicologists struggle to sort out the few available texts, vase paintings, and statuary.

The Sicilian poet, Theocritus (xvi. 45), calls the barbitos an instrument of many strings, i.e. more than seven, which the Hellenes considered a perfect number, and matched the seven strings customary in the kithara.

Anacreon (a native of Teos in Asia Minor) sings that his barbitos "only gives out erotic tones" – a remark which could have been metaphorical, but could also be a literal reference to the instrument's being tuned in the Greek harmonia called Ιαςτιαν (Iastian).

Pollux calls the barbiton a barymite instrument (from βάρυς, "heavy", and μίτος, "string"); both the literal and figurative meanings describing an instrument that produces very deep sounds. These would have been re-enforced by the barbiton's larger soundbox, compared to a kithara or a much smaller phorminx (folk lyre). The strings were twice as long as those of the phorminx and hence sounded about an octave lower.