Mamilius (fem. Mamilia) was the nomen of the gens Mamilia, a plebeian family of ancient Rome, like the Fulvii originally from Tusculum.

The Mamilii were a princely family of Tusculum, and considered themselves to be descended from the mythical Telegonus (son of Odysseus and Circe) who founded the city. Traditions included the Tusculan ruler Octavius Mamilius marrying a daughter of Lucius Tarquinius, and Lucius Mamilius saving Rome when it was attacked in 460.

The first undoubtedly historical Mamilius is Lucius Mamilius Vitulus, consul in 265 BC. Another branch that appears somewhat later is the Mamilii Turrini.