Welsh nobleman whose belated support of King Henry III of England was a major factor in the collapse of the baronial rebellion led by Simon de Montfort (Montfort, Simon de, Earl Of Leicester).

Gilbert married Alice of Angoulême, niece of King Henry III, succeeded his father (Richard de Clare) in July 1262, and joined the baronial party led by Simon de Montfort. With Simon, Gloucester was at the battle of Lewes in May 1264, when the king himself surrendered to him, and after this victory he was one of the three persons selected to nominate a council. Soon, however, he quarreled with Simon. Leaving London for his lands on the Welsh border, he met Prince Edward (afterward King Edward I) at Ludlow, just after his escape from captivity, and contributed largely to Edward's victory at Evesham in August 1265. But this alliance was as transitory as the one with Simon. Gloucester championed the barons who had surrendered at Kenilworth in November and December 1266 and, after putting his demands before the king, secured possession of London (April 1267). Gloucester quickly made his peace with Henry III and with Prince Edward. Under Edward I he spent several years fighting in Wales, or on the Welsh border. He was succeeded by his son, also named Gilbert de Clare (1291–1314), who was killed at the Battle of Bannockburn.