A mastodon (mastós 'breast' + odoús 'tooth') is any proboscidean belonging to the extinct genus Mammut. Mastodons inhabited North and Central America from the late Miocene up to their extinction at the end of the Pleistocene 10,000 to 11,000 years ago.

Mastodons are the most recent members of the family Mammutidae, which diverged from the ancestors of elephants at least 25 million years ago.

M. americanum, the American mastodon, and M. pacificus, the Pacific mastodon, are the youngest and best-known species of the genus. They lived in herds and were predominantly forest-dwelling animals. M. americanum is inferred to have had a browsing diet with a preference for woody material, distinct from that of the contemporary Columbian mammoth. Mastodons became extinct as part of the Quaternary extinction event that exterminated most Pleistocene megafauna present in the Americas, believed to have been caused by a combination of climate changes at the end of the Pleistocene and hunting by recently arrived Paleo-Indians, as evidenced by a number of kill sites where mastodon remains are associated with human artifacts.