Isabella de Forz (Latinized to de Fortibus) (July 1237 – November 10, 1293) is the eldest daughter of Baldwin de Redvers, Sixth Earl of Devon (1217-1245) and feudal baron of Plympton in Devon.
After the early death of her husband and her brother before she is thirty years old, she inherits their estates and becomes one of the richest women in England, living mainly in Carisbrooke Castle on the Isle of Wight, which she holds from the king as tenant-in-chief.
She has six children, all of whom die before her.
On her death bed, she is persuaded to sell the Isle of Wight to King Edward I, in a transaction that has ever since been considered questionable.
Her heir to the feudal barony of Plympton is her cousin Hugh II Courtenay (1276-1340), feudal baron of Okehampton, Devon, who in 1335 is declared Earl of Devon.
A weir on the River Exe is named after her, and she is the subject of at least two legends.