Guy de Thouars was the second husband of Constance, Duchess of Brittany. Guy served as regent of Brittany between 1203 and 1206 for his infant daughter Alix, Duchess of Brittany.

In 1196 Constance had been imprisoned by Ranulph de Blondeville, 4th Earl of Chester. However, the Bretons rose in revolt to protest her imprisonment and Ranulph was forced to release her. Once home in Brittany she married Guy of Thouars.
Between 1198 and the time of her death delivering twin daughters, Constance acted as regent for her young son Arthur I, Duke of Brittany. Constance had abdicated her ducal throne in Arthur's favour in 1194.

Constance died due to complications during the delivery of her twin daughters in 1201.

Once Duke Arthur I died in 1203, he was succeeded by his infant maternal sister, Alix of Thouars. Once his infant daughter inherited the throne, Guy of Thouars became regent of Brittany.

In 1206, however, Philip II of France took the regency of Brittany himself, much to the consternation of the Breton nobles. The young Arthur had already sworn fealty to Philip as king in 1199; Philip now chose this opportunity to exert direct influence in Brittany.

In 1213 Philip II of France arranged for Alix of Thouars to marry Peter of Dreux. That same year Guy de Thouars died in 1214 and was buried at Villeneuve Abbey, Nantes with his wife.