San Sebastián is a town and municipality located in the northwestern region of Puerto Rico. It is situated south of Isabela, Quebradillas, and Camuy; north of Las Marías; east of Moca and Añasco; and west of Lares. The municipality is administratively divided into twenty-four barrios plus San Sebastián Pueblo, the main urban center.
San Sebastián functions as a principal city within the Aguadilla-Isabela-San Sebastián Metropolitan Statistical Area, making it part of the larger economic and urban framework of northwestern Puerto Rico.