Italian architect whose work represents the development in style from late Mannerism to early Baroque. He was the chief Roman architect during the latter third of the 16th century and contributed to most of the major architectural projects undertaken in Rome during that period.

Della Porta was a follower of Michelangelo and continued two of his greatest architectural projects, the Piazza del Campidoglio and St. Peter's in the Vatican at Rome. Working with Domenico Fontana, the architect to Pope Sixtus V, Della Porta gave a higher, more pointed profile than Michelangelo had intended to the dome of St. Peter's; it became the prototype of the Baroque dome. He also added the facade to Giacomo da Vignola's Gesù, mother church of the Jesuits, which was widely copied by Jesuit missionaries and became the model of many Baroque church facades. Della Porta made Il Gesù's facade dramatic and lively by gradually increasing the number of architectural elements toward the centre of his design, thus creating a sense of tension released by entrance into the building's seemingly vast interior. He designed a number of palaces, the most famous being the Villa Aldobrandini (1598–1604; Frascati), his last work, notable for its huge broken pediment and elegant fenestration.