Gilbert de Clare, 4th Earl of Hertford, 5th Earl of Gloucester, 1st Lord of Glamorgan, 7th Lord of Clare (1180 – 25 October 1230) was the son of Richard de Clare, 3rd Earl of Hertford (c. 1153–1217), from whom he inherited the Clare estates. He also inherited from his mother, Amice Fitz William, the estates of Gloucester and the honour of St. Hilary, and from Rohese, an ancestor, the moiety of the Giffard estates. In June 1202, he was entrusted with the lands of Harfleur and Montrevillers.^[1]

In 1215 Gilbert and his father were two of the barons made Magna Carta sureties and championed Louis "le Dauphin" of France in the First Barons' War, fighting at Lincoln under the baronial banner. He was taken prisoner in 1217 by William Marshal, whose daughter Isabel he later married on 9 October, her 17th birthday. In 1223 he accompanied his brother-in-law, Earl Marshal, in an expedition into Wales. In 1225 he was present at the confirmation of Magna Carta by Henry III. In 1228 he led an army against the Welsh, capturing Morgan Gam, who was released the next year. He then joined in an expedition to Brittany, but died on his way back to Penrose in that duchy. His body was conveyed home by way of Plymouth and Cranborne to Tewkesbury. His widow Isabel later married Richard Plantagenet, Earl of Cornwall & King of the Romans. His own arms were: Or, three chevronels gules.

Gilbert de Clare had six children by his wife Isabel, née Marshal:^[2]