The depressor labii inferioris muscle is a four-sided facial muscle located in the jaw area that draws the lower lip down and to the side.

Originating within the lower jaw — at an area called the oblique line of the mandible — this muscle inserts into the skin of the lower lip and blends with the orbicularis oris muscle. It extends the fibers of the platysma muscle while its own fibers are combined with yellow fat.

Also known as the quadratus labii inferioris and the quadratus menti, the depressor labii inferioris allows for facial expressions, the playing of the trumpet, and kissing. It is assisted by the other muscles that lower the lip: the risorius, the depressor anguli oris and the mentalis.

Supplied by the facial nerve, ailments affecting the depressor labii inferioris include myalgia (muscle pain), stroke, myopathy (muscle disease), strains, Bell's palsy, lacerations, contusions, infectious myositis (inflammation of the muscle), tears, atrophy (loss of muscle), and neuromuscular diseases. Symptoms related to these disorders include decreased movement of the lip, loss of muscle control, paralysis, and muscle spasms.