Isabel de Clare, 4th Countess of Pembroke

Isabel de Clare, suo jure Countess of Pembroke and Striguil (1172–1220), was a Cambro-Norman-Irish noblewoman and one of the wealthiest heiresses in Wales and Ireland. She was the wife of William Marshal, 1st Earl of Pembroke, who served four successive kings as Lord Marshal of England. Her marriage had been arranged by King Richard I.

Isabel was born in 1172 in Pembrokeshire, Wales, the eldest child of Richard de Clare, 2nd Earl of Pembroke (1130 – 20 April 1176), known to history as "Strongbow", and Aoife of Leinster, who was the daughter of Dermot MacMurrough, the deposed King of Leinster and Mor Uí Thuathail. The latter was a daughter of Muirchertach Ua Tuathail and Cacht Ní Morda. The marriage of Strongbow and Aoife took place in August 1170, the day after the capture of Waterford by the Cambro-Norman forces led by Strongbow.

Isabel's paternal grandparents were Gilbert de Clare, 1st Earl of Pembroke and Isabel de Beaumont. She had a younger brother Gilbert de Striguil who, being a minor, was not formally invested with either the earldom of Pembroke or of Striguil. It is unlikely that his father could have passed on the title to Pembroke as he himself did not possess it. When Gilbert died in 1185, Isabel became Countess of Pembroke in her own right (suo jure) until her death in 1220. In this way, she could be said to be the first successor to the earldom of Pembroke since her grandfather Gilbert, the first earl. By this reckoning, Isabel ought to be called the second countess, not the fourth countess of Pembroke. In any event, the title Earl was re-created for her husband. She also had an illegitimate half-sister Basile de Clare, who married three times. Basile's husbands were: Robert de Quincy; Raymond Fitzgerald, Constable of Leinster: Geoffrey FitzRobert, Baron of Kells.