San Marcos is a city in and the county seat of Hays County, Texas, United States. The city is mostly a part of the Greater Austin metropolitan area. San Marcos's limits extend into Caldwell and Guadalupe Counties, as well. The latter is part of the Greater San Antonio Metropolitan Area. San Marcos is 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.