Borromini, Francesco, originally Francesco Castelli (1599–1667)

Swiss-born Italian baroque architect. He was one of the two most important architects (with Bernini, his main rival) in 17th-century Rome. Whereas Bernini designed in a florid, expansive style, his pupil Borromini developed a highly idiosyncratic and austere use of the classical language of architecture. His genius may be seen in the cathedrals of San Carlo alle Quattro Fontane (1637–41), Sant' Ivo della Sapienza (1643–60), and the Oratory of San Filippo Neri (1638–50).

Borromini was renowned for his brilliant and unorthodox treatment of space and light, with designs based on geometric figures rather than on the proportions of the human body.

He started as a stone cutter in Milan and went to Rome in 1620 to work first for Carlo Maderno (1556–1629), a distant relation, at St Peter's, and then on the Palazzo Barberini with Bernini. When Maderno died he continued work under Bernini, who eventually became his great rival. Borromini's highly original plan for San Carlo alle Quattro Fontane (1665–7), which had an oval dome, made his reputation throughout Europe. His later designs included the Santa Agnese in Piazza Navona (1653–7) and the facade of the Collegio di Propaganda Fide (about 1660).