The Peninj Mandible(Peninj 1), also called Natron mandible, is the fossilized lower jaw and teeth of an australopithecine specimen, likely that of Australopithecus boisei or a similar population. It was discovered in West Lake Natron, in Ngorongoro District of Arusha Region of Tanzania by Kamoya Kimeu, Glynn Isaac, and Richard Leakey in 1964.

This mandible (jaw) is estimated to be 1.5 million years old and it is characterized as having a robust build with large molars and reduced incisors. The specimen is believed to be an adult male.