Martini, Simone (circa 1280-1344), Italian painter, who was one of the most original and influential artists of the Sienese school. Simone was born in Siena. Building on the techniques for indicating three-dimensional space developed by the Sienese master Duccio di Buoninsegna, Simone added a refined contour of line, grace of expression, and serenity of mood. He painted many frescoes, introducing the fresco technique into the Sienese school. He also painted altarpiece panels, such as the Virgin and Child (1320) for the Church of Saint Catherine in Pisa.

Simone lived in Assisi for a time, where he produced one of his greatest frescoes, illustrating scenes from the life of St. Martin for the chapel of St. Martin. In 1339, at the request of Pope Benedict XII, he went to Avignon, where he executed frescoes in the papal palace and the cathedral. Among his works are Saint John the Baptist (National Gallery of Art, Washington, D.C.) and The Annunciation (1333, Uffizi Gallery, Florence), considered one of the greatest achievements of the Sienese school.