San Marcos is a city in the North County region of San Diego County, California. As of the 2020 census, the city's population was 94,833. It is the site of California State University San Marcos and Palomar College. 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, within the southwestern part of the city and its sphere of influence but is an unincorporated community.

California's 50th congressional district is a congressional district in the U.S. state of California, and encompasses parts of the Mid-Coast and northeastern parts of San Diego County. Scott Peters is currently the U.S. representative for California's 50th congressional district.