Castro Urdiales (Spanish pronunciation: [ˈkastɾowɾˈðjales]) is a seaport of northern Spain, in the autonomous community of Cantabria, situated on the Bay of Biscay.

Castro Urdiales is a modern town, although its castle and the Gothic-style parish church of Santa María de la Asunción, date from the Middle Ages.

[2] The Lolin and La Castreña anchovy canning factories serve as a reminder of the town's closeness to this industry and its proximity to the sea.

Its rapid rise in population and prosperity dates from the increased development of iron mining and railway communication which took place after 1879.

[2] The monument complex of Castro Urdiales, also known as Puebla Vieja, has medieval origins and is located near the sea.