Masaccio (born Tommaso Cassai or in some accounts Tommaso di Ser Giovanni di Mone; December 21, 1401 – autumn 1428), was the first great painter of the Quattrocento period of the Italian Renaissance. His frescoes are the earliest monuments of Humanism, and introduce a plasticity previously unseen in figure painting.

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

Despite his brief career, he had a profound influence on other artists. He was one of the first to use scientific perspective in his painting, employing techniques such as vanishing point in art for the first time. He also moved away from the Gothic style and elaborate ornamentation of artists like Gentile da Fabriano to a more naturalistic mode which employed perspective for greater realism.