Duke of Frías (Spanish: Duque de Frías) is a hereditary title in the peerage of Spain accompanied by the dignity of Grandee, created in 1492 by King Ferdinand II of Aragon and conferred to his son-in-law Don Bernardino Fernández de Velasco, 2nd Count of Haro, Constable of Castile, and Viceroy of Granada.

The House of Velasco was one of the most powerful and influential noble Castilian families of the Middle Ages and the beginning of the Modern Era.

The lineage was of distant royal origin, being the Velascos a minor branch of the Astur-Leonese dynasty, but re-elevated when Don Juan de Velasco (1368–1418), was appointed hereditary Lord High Chamberlain-Chancellor or Camarero mayor to the Kings of Castile.

His son, Don Bernardino de Velasco, 3rd Count of Haro and second hereditary Constable of Castile, received the title of Duke of Frías.

After becoming a widow, he then married Doña Juana de Aragón, illegitimate daughter of King Ferdinand II of Aragon.