Isabella of Castile, Duchess of York (1355 – 23 December 1392) was the daughter of King Peter and his mistress María de Padilla (d. 1361). She accompanied her elder sister, Constance, to England after Constance's marriage to John of Gaunt, 1st Duke of Lancaster, and married Gaunt's younger brother, Edmund of Langley, 1st Duke of York.