Sienese painter active during the 13th century. Nothing is known of his life, and his only certain work is a Madonna and Child in the Palazzo Pubblico, Siena. The picture bears an inscription with the date 1221, but this has been the subject of much controversy, as stylistically the painting seems to belong about half a century later. It has been suggested that the inscription may have some commemorative purpose, the significance of which is now lost, rather than being a record of the date of execution. Although the painting is majestic in effect and follows Byzantine conventions of iconography, the figures to some extent relax the stiff linear patterns that had been conventional in central Italian painting up to that time. On the basis of this picture a number of other panels, most of which are in the Siena Pinacoteca, have been assigned to Guido or his school. Despite his great obscurity, he is regarded as sharing with Coppo di Marcovaldo the honour of founding the Sienese School.