Texas State University–San Marcos (informally Texas State University, or simply Texas State; the use of TSU is discouraged) is an emerging research university located in San Marcos, Texas, United States. Established in 1899 as the Southwest Texas State Normal School, it opened its doors in 1903 to 303 students with a focus to educate students to become teachers. Since that time it has grown into the largest institution in the Texas State University System and the fifth-largest university in Texas boasting an enrollment of over 34,000 students. Academically, it is composed of 10 colleges and about 50 schools and departments across multiple disciplines, including nationally recognized programs in Geography, Criminal Justice and Music.

Texas State's main campus sits on 457-acre (1.85 km2) of hills along the San Marcos River. It is also has a satellite campus that started as a multi-institution teaching center offering undergraduate and graduate programs at the Texas State University Round Rock Campus (RRC) in the greater north Austin area. Additionally, the main campus in San Marcos serves as the location of the fictional school TMU (Texas Methodist University) in the NBC TV series Friday Night Lights.

Texas State University is the only university in the state to have a former President of the United States as an alumnus. President Lyndon B. Johnson graduated from what was then Southwest Texas State Teachers College in 1930 with a teaching certificate and a Bachelor of Science in history. In 1965, he returned to Southwest Texas State College, as the school was known then, to sign the Higher Education Act of 1965.