San Marcos, CA has a population of 91,479 and is the 355th largest city in the United States. The population density is 3,752 per sq mi which is 1515% higher than the California average and 4041% higher than the national average. The median age in San Marcos is 35 which is approximately 4% lower than the California average of 36. In San Marcos, 58% of the population over 15 years of age are married, 65% speak English and 26% speak Spanish. 54% of San Marcos residents were born in California, 22% were born out of state, 1% were born outside of the United States and 22% were foreign born.