Lay brother is an outdated term which referred to a male member of a religious order who has not been ordained as priests. The term was particularly used in the Catholic Church. Lay brothers were distinguished from choir monks or friars in that they did not pray in choir, and from clerics, in that they did not prepare for holy orders. This specific definition is no longer applied by the Catholic Church.

Lay brothers played a crucial role in maintaining the upkeep of facilities, performing community service work, and providing technical and administrative expertise to assist with the mission of their order, while clerics typically focus on preaching, liturgy, and leadership.

In religious institutes for women, the equivalent term was lay sister. Lay brothers were originally created to allow those who were skilled in particular crafts or did not have the required education to study for holy orders to participate in and contribute to the life of a religious order.

"In early Western monasticism, there was no distinction between lay and choir religious. The majority of St. Benedict's monks were not clerics, and all performed manual labour, the word conversi being used only to designate those who had received the habit late in life, to distinguish them from the oblati and nutriti. But, by the beginning of the 11th century, the time devoted to study had greatly increased, thus a larger proportion of the monks were in Holy Orders, even though great numbers of illiterate persons had embraced the religious life. At the same time, it was found necessary to regulate the position of the famuli, the hired servants of the monastery, and to include some of these in the monastic family. So in Italy the lay brothers were instituted; and we find similar attempts at organization at the Abbey of St. Benignus at Dijon, under William of Dijon (d. 1031) and Richard of Verdun (d. 1046), while at Hirschau Abbey, Abbot William (d. 1091) gave a special rule to the fratres barbati and exteriores."

Leslie Toke in the Catholic Encyclopedia (1910) writes that

At Cluny Abbey the manual work was relegated mostly to paid servants, but the Carthusians, the Cistercians, the Order of Grandmont, and most subsequent religious orders possessed lay Brothers, to whom they committed their secular cares. In particular, at Grandmont, the complete control of the order's property by the lay brothers led to serious disturbances, and finally to the ruin of the order; whereas the wiser regulations of the Cistercians provided against this danger and formed the model for the later orders. In England, the Benedictines made but slight use of lay brothers, finding the service of paid attendants more convenient.

Nonetheless, he adds that they are "mentioned in the customaries of the Abbey of St. Augustine at Canterbury and the Abbey of St. Peter at Westminster". Craig Lescher notes the Gilbertines, the Order of Grandmont and the Cistercians as providing historical examples of revolts carried out by lay brothers.

In 1965, the Second Vatican Council issued the document Perfectae Caritatis, which called upon all religious institutes to re-examine and renew their charism. As part of the subsequent reforms and experimentation, many of the distinctions between lay and choir religious in terms of dress and spiritual regime were abolished or mitigated. In many religious institutes, lay and choir religious wear the same habit.