(the Younger) (1458-1546) Florentine High Renaissance architect. He worked under Bramante and Peruzzi in Rome. His masterpiece is the monumental Palazzo Farnese, Rome (begun 1513, completed by Michelangelo). Sangallo took over as chief architect of St Peter's 1539 and expanded on Bramante's original plan, but it was left to Michelangelo, his successor, to make of the building a convincing whole.