A religious brother (abbreviated Br. or Bro. as a title) is a lay male member of a religious institute or religious order who commits himself to following Christ in consecrated life, usually by the vows of poverty, chastity and obedience. Equivalent to a religious sister, a religious brother typically lives in a religious community and works in a ministry appropriate to his capabilities.

A brother might practice any secular occupation. Some religious institutes are composed only of brothers; others are made up of brothers and clerics (priests or ministers, and seminarians).

In some Christian groups, members refer to each other as "brother" or "sister". For example, Shakers use the title for all adult male members.

As monasticism developed in the early days of Christianity, most monks remained laymen, as ordination to ministry was seen as a hindrance to the monks' vocation to a contemplative life. Guided by the Rule of St. Benedict, the main lifestyle they followed was either agricultural or that of a desert hermit. Various forces and trends through the Middle Ages led to the situation where monks were no longer following this manner of living. Instead, they were focusing primarily on the religious obligations of intercessory prayer, especially for donors to the monasteries. This was encouraged by a spiritual reliance among the general membership of the Catholic Church upon the prayers of monastics to achieve salvation.

One practical consequence of this situation was that the bulk of the physical work which needed to be done for the simple survival of the monastic community came to be done by men who volunteered their services on a full-time basis, and who followed a less severe regimen of prayer. Called donates or oblati, they were not considered to be monks, but they were nonetheless gradually accepted as members of the monastic community.

In other communities, a separate labor force of "lay brothers" or conversi was cultivated in order to handle the temporal business of the abbey. These men were professed members of the community but were restricted to ancillary roles of manual labor. A rigid class system emerged from this arrangement in which the clerics (priests and seminarians) exercised complete control over the lay brothers. In some cases, lay brothers received little or no formal education, could neither hold office nor vote within their communities, and were forbidden from passing from the lay to the clerical state.

In the 17th century, education of the poorer classes began to be seen as a means of providing charity, which had always been a mandate of Christianity. A leading figure of this approach was St. Jean Baptiste de la Salle, a canon of Reims Cathedral, who began to help the poor children of the city. As he was gradually drawn into education as a means for this purpose, he established a new congregation of men for this work, who were called the Institute of the Brothers of the Christian Schools. De la Salle had initially intended the Institute to be composed of both ordained and lay members, but the death of the candidates he sent to Rome for ordination while en route convinced him to keep the Institute composed only of laymen. Thus the establishment of a recognized status of "brother" as other than an agricultural laborer came to emerge in the Church.

The social devastations of the 18th and 19th centuries saw the gradual emergence of other similar congregations of men, dedicated primarily to education. Other examples of such congregations are the Marist Brothers, the Brothers of Holy Cross, the Institute of the Brothers of the Christian Schools (also known as the De La Salle Brothers), Brothers of Christian Instruction of St Gabriel (Gabrielites) and the Congregation of Christian Brothers.