San Marcos is a city in the North County region of San Diego County in the U.S. state of California. As of the 2010 census, the city's population was 83,781. It is the site of California State University San Marcos. The city is bordered by Escondido to the east, Encinitas to the southwest, Carlsbad to the west, and Vista to the northwest. Lake San Marcos is an enclave, or county island, in the southwestern part of the city, within San Marcos' sphere of influence but technically an unincorporated community.