An archdeacon is a senior clergy position in the Church of the East, Chaldean Catholic Church, Syriac Orthodox Church, Anglican Communion, St Thomas Christians, Eastern Orthodox churches and some other Christian denominations, above that of most clergy and below a bishop. In the High Middle Ages it was the most senior diocesan position below a bishop in the Catholic Church. An archdeacon is often responsible for administration within an archdeaconry, which is the principal subdivision of the diocese. The Oxford Dictionary of the Christian Church has defined an archdeacon as "A cleric having a defined administrative authority delegated to him by the bishop in the whole or part of the diocese." The office has often been described metaphorically as oculus episcopi, the "bishop's eye".

In the Latin Catholic Church, the post of archdeacon, originally an ordained deacon (rather than a priest), was once one of great importance as a senior official of a diocese. The duties are now performed by officials such as auxiliary or coadjutor bishops, the vicar general, and the episcopal vicars. The title remains.

The term "archdeacon" appears for the first time in Optatus of Mileve's history of Donatism of about 370, in which he applies it to someone who lived at the beginning of that century. From the office of the diaconus episcopi, a deacon whom the bishop selected to administer the church's finances under the bishop's personal direction, the office of archdeacon gradually developed, as certain functions were reserved to him by law. These functions included not only financial administration but also the discipline of the clergy, and examination of candidates for priesthood. From the 8th century, there was in the West a further development of the authority of the archdeacon, who now enjoyed a jurisdiction independent of the bishop.

Large dioceses had several archdeaconries, in each of which the archdeacon (usually by now a priest), had an authority comparable to that of the bishop. He was often appointed not by the bishop but by the cathedral chapter or the king. However, from the 13th century, efforts were made to limit their authority. This was effected in part by the institution of the new office of vicar general. In 1553, the Council of Trent removed entirely the independent powers of archdeacons. Those who had been in charge of different parts of the diocese gradually ceased to be appointed. Only the archdeacon associated with the cathedral chapter continued to exist as an empty title, with duties almost entirely limited to liturgical functions.

The title of archdeacon is still conferred on a canon of various cathedral chapters, and the word "archdeacon" has been defined in relation to the Latin Catholic Church as "a title of honour conferred only on a member of a cathedral chapter".

However, Eastern Catholic Churches still utilize archdeacons.

When Sixtus became Pope in 257, he ordained as deacon the young Lawrence who was only 32, the first among the seven deacons who served in the cathedral church.

Later, Sixtus appointed Lawrence as Archdeacon of Rome, the first among the seven deacons who served in the cathedral church.