Duke of Medina Sidonia is a peerage grandee title of Spain in Medina-Sidonia, holding the oldest extant dukedom in the kingdom, first awarded by King John I of Castile in 1380. His father, Henry II of Castile (c.1334-1379), had an illegitimate son named Enrique de Castilla y de Sousa with Juana de Sousa, but after being made a Duke by his half-brother in 1380, he died in 1404, without a successor. The title then returned to the Crown.