della Porta, Giacomo (c. 1537–1602)

Italian architect. One of the leading exponents of Mannerism in Rome during the late 16th century, he is best known for completing works by Michelangelo. These include the Palazzo dei Conservatori on the Capitol and, most notably, the dome of St Peter's Cathedral (1586–90), though both he and Domenico Fontana made a number of alterations to Michelangelo's designs.

Born in Rome, della Porta trained under Michelangelo and was later influenced by Vignola, developing a style based upon academic Mannerism. Sometime after 1572 he completed the facade for Vignola's Gesù, the mother church of the Jesuit order, He then incorporated features of Vignola's design into several of his own churches in Rome, including Santa Maria dei Monti (1580–81), S Atanasio (1580–83), and San Andrea della Valle (1591).