Castle of San Marcos (El Puerto de Santa María) should not be confused with Castillo de San Marcos.
| Castle of San Marcos | |
| Native Name: | Castillo de San Marcos |
| Native Language: | Spanish |
| Coordinates: | 36.5964°N -6.2266°W |
| Location: | El Puerto de Santa María, Cádiz, Spain |
| Designation1: | Spain |
| Designation1 Offname: | Castillo de San Marcos, Castillo de Alfonso X El Sabio |
| Designation1 Type: | Non-movable |
| Designation1 Criteria: | Monument |
| Designation1 Date: | 1920-08-30 |
| Designation1 Number: | (R.I.) - 51 - 0000183 - 00000 |
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.