San Marcos ( SAN MAR-kəs) is a city and the county seat of Hays County, Texas, United States. The city's limits extend into Caldwell and Guadalupe counties, as well. San Marcos is within the Austin–Round Rock metropolitan area and on the Interstate 35 corridor between Austin and San Antonio. Its population was 44,894 at the 2010 census and 67,553 at the 2020 census.
Founded on the banks of the San Marcos River, the area is thought to be among the oldest continuously inhabited sites in the Americas. San Marcos is home to Texas State University and the Meadows Center for Water and the Environment.In 2010, San Marcos was listed in Business Week's fourth annual survey of the "Best Places to Raise your Kids". In 2013 and 2014, the United States Census Bureau named it the fastest-growing city in the United States. In December 2013, it was named number nine on Business Insider's list of the "10 Most Exciting Small Cities In America".