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 2015 the U.S. Census Bureau estimated the population at 60,684 Founded on the banks of the San Marcos River, the area is thought to be among the oldest continuously inhabited sites in the Americas.