Oceanside is a city on the South Coast of California, located in San Diego County. The city had a population of 174,068 at the 2020 United States census, up from 167,086 at the 2010 census, making it the most populous city in the North County region of San Diego. The city is a popular tourist destination, owing to its historic landmarks, beaches, and architecture. Oceanside's origins date to 1798, when the Spanish founded the village of San Luis Rey with the establishment of Mission San Luis Rey de Francia by Fermín de Lasuén. During the Spanish period, Mission San Luis Rey grew to be the largest of all the Californian missions, but following the Mexican secularization act of 1833 the mission and its community declined. Following the U.S. conquest of California, the former mission lands were developed into an oceanfront resort and the community's name gradually changed to Oceanside.