San Marcos is a city located in the northern portion of San Diego County, California, USA. As of 2009, the city had a total population of 82,743. Outside the San Diego region, it is best known as the home of California State University, San Marcos. San Marcos is located at 33° 08' 31"N 117° 10' 13"W / 33.14194; -117.17028.

According to historical legends, the San Luis Rey Mission flocks were robbed by a small band of Indians in the late 1700s. Fleeing the Spanish troops, the Indians escaped to the hills. While pursuing the Indians, the Spaniards came upon a fertile valley in 1797 which was named Los Vallecitos de San Marcos (Little Valleys of Saint Mark) to honor the day of discovery: April 25th, "St. Mark’s Day." On April 22, 1840, Governor Juan Bautista Alvarado granted Los Vallecitos de San Marcos to his relative, Jose Mario Alvarado. In 1846 Indians killed Jose Alvarado, and the land was left to his wife; she then sold the land to Lorenzo Soto. In the late 1850s, Soto sold part of his land to Cave Couts and his family was soon raising livestock. Although Cave Couts owned the land, Major Gustavus French Merriam from Topeka, Kansas made the first permanent settlement. Merriam homesteaded 160 acres in the north Twin Oaks Valley and began wine and honey production.

After Major Merriam’s settlement, German and Dutch immigrants began moving into the area in the early 1880s. Then in 1883 a few miles south of the settlement, John H. Barham founded the first town in the area. By 1884, the town of Barham had a post office, blacksmith, feed store and a weekly newspaper. In 1887 the San Marcos Land Company bought almost all of the San Marcos land formerly owned by the Couts family and promptly divided the land into tracts. Soon the beautiful hills began attracting home-seekers.

The original town of San Marcos was at the intersection of what is now Grand Avenue and Rancho Santa Fe Road. In 1887 the Santa Fe Railroad announced that it was going to lay tracks going through the valley, but to the disappointment of the citizens, the tracks were laid one mile away from the center of the town. By 1896, San Marcos was a community with its own stores, post office, blacksmith and railroad depot. In 1903, the town appeared to be going downhill so the people of San Marcos picked up their homes and moved along the railroad tracks to what now are Mission Road and Pico Avenue. In 1905, the town had every convenience, including rural mail delivery and telephone service. The first school in the area, which had started in Barham in 1880, was moved in 1889 to San Marcos. Later that same year, the Richland School was built, being the second school in San Marcos. The main business in San Marcos in the 1800s and early 1900s was farming. Then in the mid-1900s, dairies and poultry production became a big part of the business in the town.

San Marcos experienced a period of growth from 1956 onward, when the first water from the Colorado River arrived. Several small businesses were founded and the population rapidly increased to 2,500. San Marcos became an incorporated city on January 28, 1963. In the 1970s, San Marcos was flourishing as the third fastest-growing city in the state with a population of 17,479 by 1980. The population continued to boom over the next two decades, reaching 33,800 in 1990 and 82,743 in 2000.