In the 15th and 16th centuries Nettuno was among the major coastal fortified centers in Lazio, of which was considered the important breadbasket: to support the walled and turreted Borgo Medievale, between 1501 and 1503 in the Borgia's period, the Forte Sangallo was built by Renaissance architect Antonio da Sangallo the Elder, as it was essential to defend the town against attacks from the sea.

[13] In 1903 the writer Gabriele D'Annunzio was a guest in the town with the actress Eleonora Duse, and wrote the opera La figlia di Iorio.

[14][15] In 1925 the Convenzione di Nettuno, between Italy and Jugoslavia, was signed in the town to regulate the conditions of the Italian citizens in Dalmatia.

American forces (5th Army) were surrounded by Germans in the caves of Pozzoli in February 1944 for a week, suffering heavy casualties.

Sights include a well-preserved old quarter, the Borgo Medievale, with medieval streets and small squares, and the Forte Sangallo mentioned above.

The church keeps also a valuable polychromed wooden statue of Our Lady of Grace, which is honoured by the town with a procession every year on the first Saturday of May.

[17] Near the eastern border of Nettuno there is Torre Astura: a fortified coastal tower of medieval origin, which overlooks a point of land; it is built over the ruins of a Roman villa with a fish pond.