The San Xavier del Bac Mission is a Spanish Catholic mission located approximately 16 kilometers south of downtown Tucson, Arizona, on the Tohono O'odham Indian Reservation. It was named in honor of Saint Francis Xavier, a Catholic missionary, pioneer and co-founder of the Society of Jesus. The mission is also known as "the place where water appears" in reference to the Santa Cruz River that runs underground.

The mission was founded in 1699 by Jesuit missionary Eusebio Francisco Kino, who often visited and preached in the area. In 1732 the German Father Segesser arrived. The original mission church was vulnerable to attacks by Apache Indians, who finally destroyed it in 1770. Because Charles III of Spain expelled the Jesuits from his domain, from 1767 the mission was administered by the Franciscans.

The current building was built with native labor, under the direction of the Franciscan fathers Juan Bautista Velderrain and Juan Bautista Llorenz in the period 1783-1797, with a loan of 7,000 pesos. Unlike other Spanish missions in Arizona, San Xavier remains active and is used primarily by the Christian community of the Tohono O'odham district, also serving the native community for which it was built.

On the outside, the construction has a Moorish-inspired design, elegant and simple. There are not many archives of the architects, builders, and craftsmen responsible for its creation, but it is known that an architect named Ignacio Gaona was also in charge, with the company of his other two brothers. The architect Gaona also designed the temple located in Heroica Caborca, Sonora, and it is known that he died in the right tower of the building, hence it is unfinished. Some of the work was provided by the local Indians, they also provided some of the craft creativity. Guests entering through the sculpted mesquite wood doors are "struck" by the freshness and dazzling colors of the paintings, carvings, frescoes and statues. The interior is lavishly decorated with ornaments, showing an artistic mix of indigenous and New Spain.

The plan of the church represents the classic Latin cross. The main hallway is separated from the sanctuary by the transept, with chapels. The dome above the transept is 16 meters high and is supported by arches and esquinches. At least three different artists painted the church's interior illustrations. It is considered the mission with the finest Spanish architecture in the United States.