public, coeducational institution of higher education in San Marcos, Texas, U.S. It is part of the Texas State University System. It offers bachelor's, and master's degrees through the Graduate College and colleges of applied arts, business administration, education, fine arts and communication, health professions, liberal arts, and science. University College manages the core curriculum for undergraduate students. A Ph.D. in geography is also offered. University units conduct research in fields such as archaeology and physical anthropology, aquifers, polymers, and the American Southwest. Off-campus research and education sites include the Aquarena Center and the 3,500-acre (1,400-hectare) Freeman Ranch. Southwest Texas State enrolls approximately 22,000 students.

The university was established by the Texas legislature in 1899 as Southwest Texas State Normal (teacher training) School; instruction began in 1903. Old Main, built in 1903, is a distinctive Victorian architectural feature of the campus. The first graduate-level course was offered in 1936, and the first graduate degree was granted the following year. The school was renamed in 1918, 1923, and 1959, before it received its present name in 1969. The campus expanded in 1979 with the purchase of San Marcos Baptist Academy. U.S. President Lyndon B. Johnson (Johnson, Lyndon B.) was an alumnus of the university.