Monjeríos (Spanish pronunciation: [mõŋ.xe.ˈɾi.os], from monjería, "cloister") were quarters within a colonial Spanish mission for the housing (under conditions of near-imprisonment) of unmarried Indigenous Californian girls and single women. Girls were taken away from their parents to the monjeríos at around the age of seven until marriage. The quarters functioned as a form of social control at the missions for conversion to Catholicism, regulation of the sexuality of girls and women, and for the rearing of Indigenous children as a labor source.

The monjeríos instituted family separation on Indigenous peoples, with reports of sexual abuse. Resistance and rebellions toward the monjeríos occurred. There were monjeríos at all of the Spanish missions in California, often multiple at a single site. There were similar quarters for Indigenous boys and single men known as jayuntes. The monjeríos were not disbanded until the secularization of the missions by the First Mexican Republic in 1834.

Monjeríos were constructed at all of the twenty-one Spanish missions in Alta California established along the California coastline along the El Camino Real. There were also monjeríos constructed elsewhere, including at Mission Santo Tomás in present-day Baja California.

The monjerío was often one of the first rooms constructed in the establishment of the missions. They were deemed high priority and frequently built in the first five years of the mission's establishment. The missionaries put their importance above many other facilities, since it often took 20 to 30 years to complete all of the buildings of the mission.

Monjeríos commonly had walls that were three to four feet (1 metre) thick and bars on the high windows, if they had windows at all. Having windows near the ceilings reduced communication between people within the interior and people on the outside. The rooms were often built in areas that could be easily watched by the mission fathers, with doors that opened to the mission's central courtyard, adjacent or across from the quarters of the missionaries.

Restricting movement and communication and maximizing surveillance were integral to the design of monjeríos within the Mission complex. Only a few monjeríos were constructed with a different design or placement, such as the one located at Mission La Purisma Concepcion in modern-day Lompoc, California.

The way in which monjeríos were constructed along with their upkeep by the missionaries led to poor conditions within the rooms themselves. Missionaries themselves often described the monjeríos as benign institutions. However, other colonial officials contradicted this framing of the conditions of the monjeríos. The seventh governor of Las Californias Diego de Borica described them as "small, poorly ventilated, and infested," further noting that the confinement also contributed to an unbearable stench from the miserable conditions and a high rate of death from spreading disease.

An unnamed visitor to Mission Santa Clara wrote that the monjeríos were "so abominably infested with every kind of filth and nastiness, as to be rendered not less offensive than degrading of the human species." Russian explorer Otto von Kotzebue wrote of his eyewitness account of the conditions at Mission Santa Clara: