This church is dedicated to Our Lady of the Sea, the patron of Santurtzi's seafaring neighbourhood of Mamariga.
Records show that in the first half of the 16th century there was a chapel dedicated to St. Eulalia at a watch-tower in Mamariga. This chapel served as a reference point, because it was visible to sailors from a great distance. As a result, many charts of that period show the name Mamariga instead of Santurce (the Spanish spelling of Santurtzi).
Popular tradition tells that around the year 1600 a figure of the Virgin Mary was found among the rocks in the sea. It was taken to the chapel of St. Eulalia, which as a result was re-dedicated to Our Lady of the Sea.
The chapel was reformed and extended several times over the years, but by the turn of the 20th century it had fallen into ruins, so it was decided to build a new one. Work began in 1903 on the new chapel, designed by Francisco Berriozabal in ashlar masonry stone, with a single nave in the Neo-Gothic style. The façade is an outstanding feature, with a three-panel design in which the central panel is the most ornately decorated, with a pointed arch supported by Corinthian columns, two rose windows and a bell tower.
In the mid 1950s, due to the increase in the population of the neighbourhood, the chapel became a parish church. At that time extensions were built onto the side walls to house a sacristy and parish offices.
Inside, the most striking feature is the Neo-Gothic altarpiece with its figure of Our Lady of the Sea flanked by the Sacred Heart and by St. Joseph. At the bottom of the altarpiece is a relief dating from 1906 that depicts the apparition.