Isabella of Castile, Duchess of York (1355 – 23 December 1392) was the daughter of King Peter and his mistress María de Padilla (d. 1361).

Isabella was the youngest of the three daughters of King Peter of Castile by his favourite mistress, María de Padilla (d.

[1] On 21 September 1371, Edward III's fourth son, John of Gaunt, 1st Duke of Lancaster, married Isabella's older sister, Constance (d. 1394), who after the death of their father in 1369 claimed the throne of Castile.

Isabella accompanied her sister to England, and on 11 July 1372, at about the age of 17, married John of Gaunt's younger brother, Edmund of Langley, 1st Duke of York, fifth son of King Edward III and Philippa of Hainault, at Wallingford, Oxfordshire, as part of a dynastic alliance in furtherance of the Plantagenet claim to the crown of Castile.

[3] As a result of her indiscretions, including an affair with King Richard II's half-brother, John Holland, 1st Duke of Exeter (d. 1400), whom Pugh terms 'violent and lawless', Isabella left behind a tarnished reputation, her loose morals being noted by the chronicler Thomas Walsingham.