Eleanor de Montfort (1252–June 1282) was the only daughter of Simon de Montfort, 6th Earl of Leicester, and became the last princess of Wales before the English Conquest in 1283.

Eleanor's father was killed when she was still a child, but he had already made an alliance with Llywelyn the Last, Prince of Gwynedd, and it had been informally agreed that Llywelyn and Eleanor would marry. However, after Simon's death, his family was forced to flee England, and Eleanor was taken by her mother into exile in France. In 1275, a marriage contract was made with Llywelyn, and Eleanor began the voyage to Wales, avoiding a land passage through England. Her ship was taken by "pirates" in the pay of her cousin, King Edward I of England, who held her in captivity, mostly at Windsor, until 1278, when he finally reached terms with Llywelyn for her release. Their marriage took place at Worcester Cathedral. Despite the age difference, their marriage is believed to have been a happy one. However, in 1282, Eleanor died after giving birth to their only child, Princess Gwenllian.