Which Baroque architect was also a mathematician?

The Baroque architect who was also a mathematician was Guarino Guarini.

Camillo Guarino Guarini (1624–1683) was an Italian Theatine priest, architect, mathematician, and writer. He became a leading figure of the Piedmontese Baroque, especially in Turin, where he designed the Palazzo Carignano, the Church of San Lorenzo, and the Chapel of the Holy Shroud.

Mathematics was not merely a side interest for Guarini. His architectural designs used complex geometry, interlocking forms, and daring structural systems, especially in domes and vaults. He also wrote mathematical and scientific works, including Euclides adauctus et methodicus, published in 1671.

A common mix-up is to choose Francesco Borromini, whose highly experimental Roman churches strongly influenced Guarini. Borromini was an architect and trained stonemason, but Guarini is the figure in this group specifically identified as a mathematician. Guarini’s architectural treatise, Architettura civile, was published posthumously in 1737 and influenced later German and Austrian Baroque architects.