The Tempietto of San Pietro in Montorio, on the Janiculum hill in Rome, is a small martyrium dedicated to St. Peter — who was believed to have been crucifixed there — designed by Donato Bramante, commissioned in 1502 by the Spanish monarchs, Ferdinand of Aragon and Isabella of Castile.

Bramante was born in Monte Adrualdo, near Urbino, in 1444. Around 1474 he moved to Milan where he designed several churches in the new antique style; in 1476 the Duke of Milan, Ludovico Sforza, commissioned him the rebuilding of the choir of Santa Maria presso San Satiro, and, as space was limited, Bramante, who was also a painter and knew well how to create the illusion of space by the use of perspective, designed a very theatrical apse in bassorilievo to trick the eye.
He then moved to Rome in 1499, working under the patronage of Cardinal Della Rovere, who would later become Pope Julius II. There he was commissioned this masterpiece, regarded as one of the finest examples of High Renaissance architecture, surely being one of the most harmonious, following all the rigorous proportions and symmetry of Classical structures despite its small scale.

It is a monopteral building and was built in the courtyard of the church of San Pietro in Montorio (designed by either Baccio Pontelli, as stated by Vasari, or Meo del Caprino). Originally, according to Sebastiano Serlio, Bramante had designed the courtyard to be circular and colonnaded; however, plans were changed and the Tempietto was built in a rectangular space.
The internal diametre is about 4.5 metres; the dome itself is an emisphere (like the Pantheon’s dome), being as tall as its radius, and its drum is of the same exact height. Everything stands on a Doric tablature borrowed from the ancient Theater of Marcellus and a colonnade of sixteen slender Doric columns on a base of 3+3 steps.

The Tempietto is considered a sculpture rather than a building, being so small it can only welcome about fifteen people in its cylindrical cella.
It has clearly inspired both Bramante himself for his later design of San Peter’s cupola (unbuilt, as eventually Michelangelo’s design was chosen) and Christopher Wren for the design of the dome of St. Paul’s Cathedral in London.
