The Triangular Forum, which takes its name from its unique shape, stands on a ridge of lava rock that overlooked the valley and the mouth of the river Sarno, and preserves one of the oldest sacred areas in the city dating back to the 6th century BC.

It was accessed from via dei Teatri, through a hallway with six columns that formed the monumental façade, preceded by a public fountain. The inner portico was built in the 2nd century BC and surrounds the area of the Doric Temple with tuff columns.

A double rectangular enclosure before the stairs to the temple has been interpreted as the tomb of the legendary founder of the city, Heracles. A well is found behind the closed off area, surrounded by a circular building (tholus) with Doric columns, built by the Samnite magistrate Numerius Trebius. The Samnite Palaestra opened on the eastern side.

Date of excavation: 1765; 1767-1768; 1813; 1899; 1905; 1931; 1981-1996.