The Order of Saint Augustine, abbreviated OSA, is a religious mendicant order of the Catholic Church. It was founded in 1244 by bringing together several eremitical groups in the Tuscany region who were following the Rule of Saint Augustine, written by Saint Augustine of Hippo in the fifth century.

Mendicant orders are, primarily, certain Roman Catholic religious orders that have adopted for their male members a lifestyle of poverty, traveling, and living in urban areas for purposes of preaching, evangelization, and ministry, especially to the poor. At their foundation these orders rejected the previously established monastic model. This model prescribed living in one stable, isolated community where members worked at a trade and owned property in common, including land, buildings and other wealth. By contrast, the mendicants avoided owning property at all, did not work at a trade, and embraced a poor, often itinerant lifestyle. They depended for their survival on the goodwill of the people to whom they preached. The members of these orders are not called monks but friars.