The ministry of a deaconess is, in modern times, a usually non-ordained ministry for women in some Protestant, Oriental Orthodox, and Eastern Orthodox churches to provide pastoral care, especially for other women, and which may carry a limited liturgical role as well. The word comes from the Greek diakonos (διάκονος), for "deacon", which means a servant or helper and occurs frequently in the Christian New Testament of the Bible. Deaconesses trace their roots from the time of Jesus Christ through to the 13th century in the West. They existed from the early through the middle Byzantine periods in Constantinople and Jerusalem; the office may also have existed in Western European churches. There is evidence to support the idea that the diaconate including women in the Byzantine Church of the early and middle Byzantine periods was recognized as one of the major non-ordained orders of clergy.

The English separatists unsuccessfully sought to revive the office of deaconesses in the 1610s in their Amsterdam congregation. Later, a modern resurgence of the office began among Protestants in Germany in the 1840s and spread through Nordic States, Netherlands, United Kingdom and the United States. Lutherans were especially active and their contributions are seen in numerous hospitals. The modern movement reached a peak about 1910, then slowly declined as secularization undercut religiosity in Europe and the professionalization of nursing and social work offered other career opportunities for young women. Deaconesses continue to serve in Christian denominations such as Lutheranism and Methodism, among others. Before they begin their ministry, they are consecrated as deaconesses.

Non-clerical deaconesses should not be confused with women ordained deacons such as in the Anglican churches, the Methodist churches, and the Protestant Church in the Netherlands, many of which have both ordained deacons and consecrated deaconesses; in Methodism, the male equivalent to female deaconesses are Home Missioners.