Castle of San Marcos (also Castillo de Alfonso X El Sabio) is a medieval castle located in El Puerto de Santa María, Cádiz, Spain.

The castle was erected as a fortified church by King Alfonso X of Castile.

It was built on the site of a mosque of which the wall of the qibla survives.

Close to the castle there is a replica of the map of Juan de la Cosa, along with an explanatory plaque, and a little fountain.

The castle was declared a national monument on 30 August 1920 and is currently listed in the Spanish heritage register as a Bien de Interés Cultural.