Originally he was an Italic god paired with Salacia, possibly the goddess of the salt water. At an early date (399 BC) he was identified with Poseidon, when the Sibylline books ordered a lectisternium in his honour (Livy v. 13). In earlier times it was the god Portunes or Fortunus who was thanked for naval victories, but Neptune supplanted him in this role by at least the first century BC, when Sextus Pompeius called himself "son of Neptune". Neptune was associated as well with fresh water, as opposed to Oceanus, god of the world-ocean. Like Poseidon, Neptune was also worshipped by the Romans as a god of horses, under the name Neptune Equester, patron of horse-racing. The planet Neptune was named after the god, as its deep blue gas clouds gave early astronomers the impression of great oceans.
His festival, Neptunalia, at which tents were made from the branches of bushes, took place on July 23. He had two temples in Rome. The first, built in 25 BC, stood near the Circus Flaminius, the Roman racetrack, and contained a famous sculpture of a marine group by Scopas. The second, the Basilica Neptuni, was built on the Campus Martius and dedicated by Agrippa in honour of the naval victory of Actium.