The Spanish missions in California (Spanish: Misiones españolas en California) formed a series of 21 religious outposts or missions established between 1769 and 1833 in what is now the U.S. state of California. The missions were established by Catholic priests of the Franciscan order to evangelize indigenous peoples backed by the military force of the Spanish Empire. The missions were part of the expansion and settlement of New Spain through the formation of Alta California, expanding the empire into the most northern and western parts of Spanish North America. Civilian settlers and soldiers accompanied missionaries and formed settlements like the Pueblo de Los Ángeles.

Indigenous peoples were forced into settlements called reductions, disrupting their traditional way of life and negatively affecting as many as one thousand villages. European diseases spread in the close quarters of the missions, causing mass death. Abuse, malnourishment, and overworking were common. At least 87,787 baptisms and 63,789 deaths occurred. Indigenous peoples often resisted and rejected conversion to Christianity. Some fled the missions while others formed rebellions. Missionaries recorded frustrations with getting indigenous people to internalize Catholic scripture and practice. Indigenous girls were taken away from their parents and housed at monjeríos. The missions' role in destroying Indigenous culture has been described as cultural genocide.

By 1810, Spain's king had been imprisoned by the French, and financing for military payroll and missions in California ceased. In 1821, Mexico achieved independence from Spain, yet did not send a governor to California until 1824. The missions maintained authority over indigenous peoples and land holdings until the 1830s. At the peak of their influence in 1832, the coastal mission system controlled approximately one-sixth of Alta California. The First Mexican Republic secularized the missions with the Mexican Secularization Act of 1833, which emancipated indigenous peoples from the missions. The missions were closed down, their priests mostly returned to Mexico. The churches ended religious services and fell into disrepair. The farmlands were seized and were largely given to settlers and soldiers, along with a minority of indigenous people.

The surviving mission buildings are the state of California's oldest structures and most-visited historic monuments, many of which were restored after falling into near disrepair in the early 20th century. They have become a symbol of California, appearing in many movies and television shows, and are an inspiration for Mission Revival architecture. Concerns have been raised by historians and Indigenous peoples of California about the way the mission period in California is taught in educational institutions and memorialized. The oldest European settlements of California were formed around or near Spanish missions, including the four largest: Los Angeles, San Diego, San Jose, and San Francisco. Santa Barbara, and Santa Cruz were also formed near missions, and the historical imprint reached as far north as Sonoma in what became the wine country.

Prior to 1754, grants of mission lands were made directly by the Spanish Crown. But, given the remote locations and the inherent difficulties in communicating with the territorial governments, he delegated authority to make grants to the viceroys of New Spain. During the reign of King Charles III, they granted lands to allow establishing the Alta California missions. They were motivated in part by presence of Russian fur traders along the California coast in the mid-1700s.

The missions were to be interconnected by an overland route which later became known as the Camino Real (Royal Road). The detailed planning and direction of the missions was to be carried out by Friar Junípero Serra, O.F.M. (who, in 1767, along with his fellow priests, had taken control over a group of missions in Baja California Peninsula previously administered by the Jesuits). After Serra's death, Rev. Fermín Francisco de Lasuén established nine more mission sites, from 1786 through 1798; others established the last three compounds, along with at least five asistencias (mission assistance outposts).

Work on the coastal mission chain was concluded in 1823, completed after Serra's death in 1784. Plans to build a twenty-second mission in Santa Rosa in 1827 were canceled.^{[citation needed]}

The Rev. Pedro Estévan Tápis proposed establishing a mission on one of the Channel Islands in the Pacific Ocean off San Pedro Harbor in 1784, with either Santa Catalina or Santa Cruz (known as Limú to the Tongva residents) being the most likely locations, the reasoning being that an offshore mission might have attracted potential people to convert who were not living on the mainland, and could have been an effective measure to restrict smuggling operations. Governor José Joaquín de Arrillaga approved the plan the following year; however, an outbreak of sarampión (measles) killing some 200 Tongva people coupled with a scarcity of land for agriculture and potable water left the success of such a venture in doubt, so no effort to found an island mission was ever made.^{[citation needed]}