The San Luis Valley is a region in south-central Colorado with a small portion overlapping into New Mexico. The valley is approximately 122 miles (196 km) long and 74 miles (119 km) wide, making it the largest alpine valley in the world. It extends from the Continental Divide on the northwest rim into New Mexico on the south. It contains 6 counties and portions of 3 others. It is an extensive high-elevation depositional basin of approximately 8,000 square miles (21,000 km²) with an average elevation of 7,664 feet (2,336 m) above sea level. The valley is a section of the Rio Grande Rift and is drained to the south by the Rio Grande, which rises in the San Juan Mountains to the west of the valley and flows south into New Mexico. The San Luis Valley has a cold desert climate but has substantial water resources from the Rio Grande and groundwater.

The San Luis Valley was ceded to the United States by Mexico following the Mexican–American War. Hispanic settlers began moving north and settling in the valley after the United States made a treaty with the Utes and established a fort in the early 1850s. Prior to the Mexican war the Spanish and Mexican governments had reserved the valley to the Utes. Later in the 19th century Anglo settlers began to settle in the valley and engaged in mining, ranching, and irrigated agriculture. Today the valley has a diverse Anglo and Hispanic population.

Prior to 1868 the Capote (Kapota) band of Ute Indians lived in the valley. The Utes made a treaty of peace with the United States in 1849 after the Mexican War. Shortly thereafter settlers from New Mexico established several small settlements in what is now Colorado and in 1868 the Utes were removed to a reservation in western Colorado. They continued to play a role in Saguache in the northwestern corner of the valley from the Los Pinos Agency to the west of Saguache until they lost their extensive reservation as the result of the Meeker Massacre in 1879.

The area was administered as part of the Spanish, later Mexican, province of Nuevo Mexico. In 1843, the government of New Mexico created the Sangre de Cristo Land Grant consisting of 1,000,000 acres (4,000 km²) of the valley and granted the family of Carlos Beaubien. The area was ceded to the United States in 1848 after the Mexican–American War in the Treaty of Guadalupe Hidalgo.

Extensive settlement began in the San Luis Valley, primarily by Hispanic farmers and ranchers from New Mexico, in the 1850s. Early settlers built a church in the village that is now called San Luis and dedicated it on the Feast of Saint Louis, 21 June 1851. At present, the San Luis Valley has the largest native Hispanic population in Colorado; many families are directly descended from the original New Mexican settlers. The surge of immigration followed the construction by the U.S. Army of Fort Massachusetts for protection against the Utes, who had previously barred settlers. The history of the U.S. military presence in the valley is preserved at Fort Garland and other historic preserves in the valley.

The San Luis Valley became part of the Territory of Colorado in 1861. The original Ute population was confined to the Southern Ute and Ute Mountain Indian reservations in the late 19th century.

The San Luis Valley is the broad, generally flat, valley at the headwaters of the Rio Grande in south central Colorado and far north central New Mexico. The northern portion of the San Luis Valley is an endorheic basin; surface water does not exit this area. Irrigated agriculture is possible in the area due to groundwater and streams fed by the average 100 inches of snow the surrounding mountain ranges receive. The southern portion is drained by the Rio Grande.

There is no clear southern boundary but the term is generally used to include the San Luis Hills of southern Colorado and the Taos Plateau of northern New Mexico. About 50 miles from east to west and about 150 miles from north to south, the valley is bounded on the east by the Sangre de Cristo Mountains and on the west by the San Juan Mountains.