San Marcos is a city in the U.S. state of Texas, within the Austin–Round Rock–San Marcos metropolitan area. The city is on the Interstate 35 corridor between Austin and San Antonio and is the seat of Hays County. The city limits extend into Caldwell and Guadalupe Counties as well. Its population was 44,894 in 2010. In 2014 the U.S. Census Bureau estimated the population at 58,892, a 7.9% increase from 2013.