The phorminx (in Ancient Greek φόρμιγξ) was one of the oldest of the Ancient Greek stringed musical instruments, intermediate between the lyre and the kithara. It consisted of two to seven strings, richly decorated arms and a crescent-shaped sound box. It mostly probably originated from Mesopotamia.

While it seems to have been common in Homer's day, accompanying the rhapsodes, it was supplanted in historical times by the seven-stringed kithara. Nevertheless, the term phorminx continued to be used as an archaism in poetry.

The term phorminx is also sometimes used in ancient an modern writing to refer to all four instruments the lyre family collectively:

• barbitos
• kithara
• lyra
• phorminx

Musical instruments | Ancient music | Phorminx