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

Hays County is a county in the central portion of the U.S. state of Texas. It is part of the Austin-Round Rock metropolitan area. As of the 2020 census, its official population had reached 241,067. The county seat is San Marcos. Hays, along with Comal and Kendall Counties, was listed in 2017 as one of the nation's fastest-growing counties with a population of at least 10,000. From 2015 to 2016, Hays County, third on the national list, had nearly 10,000 new residents during the year.