Octavius Mamilius (died 498 BC) was princeps ("leader, prince") of Tusculum, an ancient city of Latium. He was the son-in-law of Lucius Tarquinius Superbus, the seventh and last king of Rome. According to tradition, the gens Mamilia was descended from Mamilia, reputedly a granddaughter of Ulysses (Odysseus) and Circe. Titus Livius described Octavius as head of one of the most distinguished families of Latium, and thus an important ally of Tarquinius.