Mission San Xavier del Bac (Spanish: La Misión de San Xavier del Bac) is an historic Spanish Catholic mission located about 10 miles (16 km) south of downtown Tucson, Arizona, on the Tohono O'odham Nation San Xavier Indian Reservation. The mission was founded in 1692 by Padre Eusebio Kino[1] in the center of a centuries-old settlement of the Sobaipuri O'odham, a branch of the Akimel or River O'odham located along the banks of the Santa Cruz River. The mission was named for Francis Xavier, a Christian missionary and co-founder of the Society of Jesus (Jesuit Order) in Europe. The original church was built to the north of the present Franciscan church. This northern church or churches served the mission until it was razed during an Apache raid in 1770.

The mission that survives today was built between 1783 and 1797, which makes it the oldest European structure in Arizona. Labor was provided by the O'odham.[1] An outstanding example of Spanish Colonial architecture in the United States, the Mission San Xavier del Bac hosts some 200,000 visitors each year.[1] It is a well-known pilgrimage site, with thousands visiting each year on foot[4] and on horseback, some among ceremonial cavalcades or cabalgatas.

The site is also known in the O'odham language as "goes in" or "comes in", meaning "where the water goes in", as the water in the Santa Cruz River came up to the surface a few miles south of Martinez Hill and then submerged again near Los Reales Wash. The Santa Cruz River that used to run year-round in this section was once critical to the community's survival, but now runs only part of the year.