Cabot, Sebastian (1475?–1557), an Italian-born navigator and mapmaker. He was a son of John Cabot and probably was born in Venice. While in the service of England, Cabot is believed to have led a seaborne expedition in search of the Northwest Passage, 1508–09. He was employed by Henry VIII as a mapmaker in 1512. In 1518 Cabot was appointed chief navigator for Spain. During a five-year voyage to the New World for Spain, 1525–30, he explored South America's La Plata River, which he named. In 1551 he became governor of England's Company of Merchant Adventurers, which opened Russia to English trade.