Servius Tullius was the legendary sixth king of Rome, and the second of its Etruscan dynasty. He reigned from 578 to 535 BC. Roman and Greek sources describe his servile origins and later marriage to a daughter of Lucius Tarquinius Priscus, Rome's first Etruscan king, who was assassinated in 579 BC. The constitutional basis for his accession is unclear; he is variously described as the first Roman king to accede without election by the Senate, having gained the throne by popular and royal support; and as the first to be elected by the Senate alone, with support of the reigning queen but without recourse to a popular vote.Several traditions describe Servius' father as divine. Livy depicts Servius' mother as a captured Latin princess enslaved by the Romans; her child is chosen as Rome's future king after a ring of fire is seen around his head. The Emperor Claudius discounted such origins and described him as an originally Etruscan mercenary, named Mastarna, who fought for Caelius Vibenna.Servius was a popular king, and one of Rome's most significant benefactors. He had military successes against Veii and the Etruscans, and expanded the city to include the Quirinal, Viminal and Esquiline hills. He is traditionally credited with the institution of the Compitalia festivals, the building of temples to Fortuna and Diana and, less plausibly, the invention of Rome's first true coinage.

Despite the opposition of Rome's patricians, he expanded the Roman franchise and improved the lot and fortune of Rome's lowest classes of citizens and non-citizens. According to Livy, he reigned for 44 years, until murdered by his daughter Tullia and son-in-law Lucius Tarquinius Superbus. In consequence of this "tragic crime" and his hubristic arrogance as king, Tarquinius was eventually removed. This cleared the way for the abolition of Rome's monarchy and the founding of the Roman Republic, whose groundwork had already been laid by Servius' reforms.