Triangularis, a name based on its shape, (also known as Depressor anguli oris) originates in the mandible and platysma and inserts in the skin and orbicular muscle at corner of the mouth. It is a muscle whose evolutionary connection to the platysma is evident, being continuous with it and extending to the mouth. This muscle causes the corners of the mouth to turn down and form the lips into an inverted U, an action stereotyped as indicating grief. It produces a frown in the mouth.

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