Masaccio (UK: , US: , Italian: [maˈzattʃo]; December 21, 1401 – summer 1428), born Tommaso di Ser Giovanni di Simone, was a Florentine artist who is regarded as the first great Italian painter of the Quattrocento period of the Italian Renaissance. According to Vasari, Masaccio was the best painter of his generation because of his skill at imitating nature, recreating lifelike figures and movements as well as a convincing sense of three-dimensionality. He employed nudes and foreshortenings in his figures. This had seldom been done before him.

The name Masaccio is a humorous version of Maso (short for Tommaso), meaning "clumsy" or "messy" Tom. The name may have been created to distinguish him from his principal collaborator, also called Maso, who came to be known as Masolino ("little/delicate Tom").

Despite his brief career, he had a profound influence on other artists and is considered to have started the Early Italian Renaissance in painting with his works in the mid- and late-1420s. He was one of the first to use linear perspective in his painting, employing techniques such as vanishing point in art for the first time. He moved away from the International Gothic style and elaborate ornamentation of artists like Gentile da Fabriano to a more naturalistic mode that employed perspective and chiaroscuro for greater realism.

Masaccio died at the age of twenty-six and little is known about the exact circumstances of his death. Upon hearing of Masaccio’s death, Filippo Brunelleschi said: "We have suffered a great loss."