religious order founded by St. Peter Nolasco (Peter Nolasco, Saint) in Spain in 1218, for the purpose of ransoming Christian captives from the Moors. It was originally a military order.

St. Raymond of Penafort (Raymond of Peñafort, Saint), Nolasco's confessor and the author of the order's rule, based the rule on that of St. Augustine (Augustine, Saint). In addition to the usual three religious vows of poverty, chastity, and obedience, the Mercedarians took a fourth vow, to offer themselves as hostages for Christian prisoners in danger of losing their faith.

The habit of a Mercedarian is white, originally to facilitate entrance into Muslim territory, and he wears a wide leather belt with a chain, suggesting the sword that all members once customarily carried.

Pope Gregory IX approved the order in 1235, and it spread rapidly through Europe. During the founder's lifetime, the order freed 2,700 prisoners and, overall, claimed to have freed about 70,000 prisoners. In 1265 a second order of Mercedarians for women was founded in Spain by St. Mary de Cervello.

In 1318 Pope John XXII decreed that the leader of the order should be a priest, an action that caused lay knights to leave the Mercedarians and join a military order of Our Lady of Montesa. The Mercedarians subsequently became a mendicant order. Mercedarians accompanied Columbus to America and founded monasteries in Latin America. They also established themselves in Africa, Italy, France, and Ireland.

In 1602 a reform movement led by Juan Bautista Gonzalez resulted in the Discalced Mercedarians, whose rule was approved in 1606 by Pope Paul V. The anticlerical mood of the 19th century came close to extinguishing the Mercedarians. In 1880, however, Pedro Armengol Valenzuela became master general, revised their constitution, and guided the order to educational, charitable, and social work, activities which the Mercedarians continued to pursue in the 20th century.