San Marcos is a city in Texas, USA. The population was 34,733 at the 2000 census. It is the county seat of Hays County. Texas State University-San Marcos (formerly Southwest Texas State University) is located in the city. San Marcos is the only habitat for the endangered Texas Blind Salamander (Eurycea rathbuni). The Aquarena Springs ecological education center is located in San Marcos. As of the census of 2000, there were 34,733 people, 12,660 households, and 5,380 families residing in the city. The population density was 736.4/km² (1,907.5/mi²). There were 13,340 housing units at an average density of 282.8/km² (732.6/mi²). The racial makeup of the city was 72.55% White, 5.53% African American, 0.65% Native American, 1.23% Asian, 0.11% Pacific Islander, 17.03% from other races, and 2.90% from two or more races. Hispanic or Latino of any race were 36.50% of the population.

San Marcos is located at 29°52′46″N, 97°56′20″W (29.879387, -97.938829) in an area locally referred to as Central Texas. This is 29 miles southwest of Austin and 47 miles northeast of San Antonio. Interstate 35 is the main highway through the town and the spring-fed San Marcos River is a notable water feature. The city is also situated on the Balcones Fault. The eastern part of the city is flat while the western part consists of rolling hills. According to the United States Census Bureau, the city has a total area of 47.4 km² (18.3 mi²). Land constitutes 47.2 km² (18.2 mi²), and 0.3 km² (0.1 mi²) of it (0.60%) is water.