The church of San Carlo alle Quattro Fontane, also called San Carlino, is a Roman Catholic church in Rome, Italy. The church was designed by the architect Francesco Borromini and it was his first independent commission. It is an iconic masterpiece of Baroque architecture, built as part of a complex of monastic buildings on the Quirinal Hill for the Spanish Trinitarians, an order dedicated to the freeing of Christian slaves. He received the commission in 1634, under the patronage of Cardinal Francesco Barberini, whose palace was across the road. However, this financial backing did not last and subsequently the building project suffered various financial difficulties. It is one of at least three churches in Rome dedicated to San Carlo, including San Carlo ai Catinari and San Carlo al Corso.

Francesco Borromini, byname of Francesco Castelli, was an Italian architect born in the modern Swiss canton of Ticino who, with his contemporaries Gian Lorenzo Bernini and Pietro da Cortona, was a leading figure in the emergence of Roman Baroque architecture.