Ancus Marcius (640 BC-616 BC.), fourth of the Kings of Rome, and possibly legendary. Like Numa, his reputed grandfather, he was a friend of peace and religion, but was obliged to make war to defend his territories. He conquered the Latins, and a number of them he settled on the Aventine Hill formed the origin of the Plebeians. He fortified the Janiculum, threw a wooden bridge across the Tiber, founded the port of Ostia, established salt-works and built a prison.

Ancus Marcius is merely a duplicate of Numa, as is shown by his second name, Numa Marcius, the confidant and pontifex of Numa, being no other than Numa Pompilius himself, represented as priest. The identification with Ancus is shown by the legend which makes the latter a bridge-builder (pontifex), the constructor of the first wooden bridge over the Tiber. It is in the exercise of his priestly functions that the resemblance is most clearly shown. Like Numa, Ancus died a natural death. He was succeeded by Lucius Tarquinius Priscus.