San Marcos, TX has a population of 57,743 and is the 690th largest city in the United States. The population density is 1,711 per sq mi which is 1665% higher than the Texas average and 1788% higher than the national average. The median age in San Marcos is 24 which is approximately 31% lower than the Texas average of 34. In San Marcos, 26% of the population over 15 years of age are married, 73% speak English and 24% speak Spanish. 71% of San Marcos residents were born in Texas, 20% were born out of state, 2% were born outside of the United States and 7% were foreign born.