California State University San Marcos (CSUSM or Cal State San Marcos) is a public university in San Marcos, California, United States. It was founded in 1989 as the 21st campus in the California State University (CSU) system.

CSUSM offers 43 bachelor's degree programs, 24 master's degree programs, 8 credential programs, and 1 joint doctoral with the University of California, San Diego.

CSUSM is accredited by the Western Association of Schools and Colleges (WASC) and also Accreditation Board for Engineering and Technology (ABET). The university had a total undergraduate enrollment of 15,431 students in fall 2024. In 2024, the university had 284 tenured faculty. The Cal State San Marcos Cougars compete in NCAA Division II as a member of the California Collegiate Athletic Association.