Castle of San Marcos (El Puerto de Santa María)

{{distinguish|Castillo de San Marcos}}

{{Infobox Historic Site

| name = Castle of San Marcos

| native_name = Castillo de San Marcos

| native_language = Spanish

| image = Castillo de San Marcos, El Puerto de Santa María, España, 2015-12-08, DD 16.JPG

| caption = Front view of Castle of San Marcos

| locmapin = Spain

| coordinates = {{coord|36|35|47.04|N|6|13|35.91|W|display=inline,title}}

| location = El Puerto de Santa María, Cádiz, Spain

| area =

| built =

| architect =

| architecture =

| governing_body =

| 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.