The Forum of the Marine Gate (44 x 39.5 m.) is located just outside this city gate, to the east of the Decumanus and directly to the south of the city wall. A triple entrance is on the Decumanus. The central passage is decorated with two marble columns. On three sides the square is surrounded by a colonnade with a floor of white marble (some missing columns have today been replaced by cypresses). In the centre of the perimetral side walls are small rectangular rooms. In the centre of the back wall is a large room with a slightly curved back wall. The floor was decorated with polychrome marble, for the most part removed in antiquity. Secondary reinforcing piers in the corners supported a cross-vault.

The walls are of Hadrianic opus mixtum. The area may have had a religious purpose. A masonry structure on the back part of the square may have been an altar.