Depressor labii inferioris (also known as quadratus labii inferioris) originates in the chin part of the mandible and inserts into the skin of the lower lip and muscles of the lower lip (o. oris). Its action pulls the red parts of the lower lip relatively straight down and the skin of the lower lip somewhat laterally and down, baring the lower teeth. It has been suggested by some to express irony.

Depressor labii inferioris is innervated by the mandibular and buccal branches of the facial nerve (VII) and is supplied with blood by the facial artery.