San Marcos is located in the state of Texas, and is the seat of Hays County. Located within the Austin–Round Rock–San Marcos metropolitan area, the city is located on the Interstate 35 corridor—between Austin and San Antonio.

Founded on the banks of the San Marcos River, the area is considered to be among the oldest continuously inhabited sites in the Northern Hemisphere.

As of the U.S. census of 2010, the population was 44,894 people in the city. The median income for a household in the city was $25,809, and the median income for a family was $37,113. Males had a median income of $25,400 versus $22,953 for females. The per capita income for the city was $13,468. About 13.8% of families and 28.5% of the population were below the poverty line, including 22.1% of those under age 18 and 15.1% of those age 65 or over.