Bastiano di Bartolo Mainardi (1466–1513) was an Italian painter of the Early Renaissance. He was born in San Gimignano and was active there and in Florence.
According to Giorgio Vasari, Mainardi is portrayed in the frescoes in the Sassetti and Tornabuoni Chapels by Domenico Ghirlandaio, Mainardi's brother-in-law and master. Vasari also claimed that Mainardi took part in Ghirlandaio's frescoes (1476) in the Abbey of Passignano in Val di Pesa, near Florence, and in the chapel of Saint Fina in the Collegiata of San Gimginano (1485). The Annunciation fresco in the loggia of San Gimignano's Collegiata, dated 1482, is often also attributed to Mainardi. Mainardi's authorship of these works was, however, proven impossible when Italian art historian Lisa Venturini discovered Mainardi's birthdate as 1466 (it was previously placed around 1460 or earlier). Thus, Mainardi was too young to have assisted Ghirlandaio in these works or to have painted the Annunciation San Gimignano (which is now regarded as a work by Domenico Ghirlandaio's brother, Davide). Further, Mainardi could probably not have painted most of the Ghirlandaio workshop paintings once attributed to him, like the famous tondo of the Madonna and Child with the Infant Saint John the Baptist and Three Angels at the Louvre.