ci.san-marcos.tx.us San Marcos is a city in the U.S. state of Texas, and is the seat of Hays County. Located within the Austin–Round Rock–San Marcos metropolitan area, the city is located on the Interstate 35 corridor—between Austin and San Antonio. Founded on the banks of the San Marcos River, the area is considered to be among the oldest continuously inhabited sites in the Northern Hemisphere. San Marcos is home to Texas State University–San Marcos, and the Aquarena Center. The population was 50,371 in 2008. San Marcos was listed in the 2010 Business Week's fourth annual survey of the "Best Places to Raise your Kids."