San Sebastián (Spanish pronunciation: [san seβasˈtjan]) is a municipality of Puerto Rico located northwest of the island, south of Isabela and Quebradillas; north of Las Marías; east of Moca and Añasco; and west of Lares. San Sebastián is spread over twenty-four wards and San Sebastián Pueblo (The downtown area and the administrative center of the city). It is a principal city of the Aguadilla-Isabela-San Sebastián Metropolitan Statistical Area.

The permission to found the town was officially given in 1752, under the leadship of the founder, Captain Cristóbal González de la Cruz, who among other neighbors had an interest in converting some cow farms into an agricultural village. The foundation of the town from the religious aspect, was consummated in December 1762 by Mariano Martin, the island catholic bishop at that time. At the beginning, by 1700, San Sebastian was a conglomerate of a few cow farms, owned by some residents of the Partido de Aguada. Las Vegas was the former plain site of one of the first cow farms located by the Guatemala riverside at the north. Also, another of those cow farms was Pepinito (today's downtown) that was a low green mountain with a white calcium carbonate face. On the north side of the town it can be seen some of this "pepinos" (cucumbers). These two cow farms gave the town its first name. From these geographical accidents come the first names of the new village: Las Vegas del Pepino (Cucumber Fields). In 1865 it is documented as San Sebastián de Las Vegas del Pepino.

At the beginning of the 19th century, wealthy Spanish families arrived in Pepino, fleeing the revolutions of Venezuela and the Dominican Republic. Later, by 1850, several families from Catalonia and the Basque Country in Spain joined the large number of isleños (Canary Islanders) that had made El Pepino their home. These people, after taking over the local political power, developed a coffee industry and brought some material progress to the town. The resident Basques in the municipality, in remembrance of their home region and its religious patron, saw the need of upgrading the old traditional Pepino used by the Canary Islanders to the new and "up-dated" San Sebastián, promoted and thus got the name formally changed by the central government authority. Nevertheless, the citizens of San Sebastián are called "pepinianos".

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