The phorminx (Ancient Greek: φόρμιγξ) was one of the earliest stringed musical instruments used in Ancient Greece. It belonged to the yoke lutes family, which came after the lyre and before the kithara. The instrument had two to seven strings, ornate parts, and a sound box shaped like a crescent moon. It likely began in Mesopotamia. During the time of Homer, the phorminx was often used by rhapsodes, who were storytellers. However, over time, it was replaced by the seven-stringed kithara. Even so, the word "phorminx" remained in use in poetry as a way to refer to older times.

The term "phorminx" is also sometimes used in both ancient and modern writings to describe all four instruments in the lyre family together.