The doors of St Davids Cathedral will be locked for up to three days as the new bishop is elected.
The building will be closed to the public as a College of representatives from across Wales will nominate and vote for the next Bishop of St Davids.
The new bishop will be the 129th Bishop of St Davids, a diocese which takes in the west Wales counties of Pembrokeshire, Carmarthenshire and Ceredigion. The election follows the retirement of Wyn Evans, who served as Bishop of St Davids for eight years.
The Electoral College will have up to three days to make a decision. Once a decision is made, the Archbishop of Wales, Dr Barry Morgan, will unlock and open the west door of the Cathedral and announce the name of the Bishop-Elect from the doorstep.
The College is made up of representatives from all six Welsh dioceses. St Davids diocese is represented by six lay people and six clergy, and the other five dioceses by three lay people and three clergy each, plus the five remaining bishops.
The process is confidential. The College nominates candidates at a meeting and then each member vote by ballot. A candidate that receives more than a two-third majority vote is declared Bishop-Elect. If no candidate reaches this majority, the process starts again.
After a bishop is elected, he or she will have 28 days to accept the role. If they accept, the election will be formally confirmed at a ceremony on January 21 at Llandaff Cathedral.
The new Bishop will be eligible for the role of Archbishop after Dr Barry Morgan retires next year. It is unlikely however, due to the new Bishop's lack of experience in the role.
The Church in Wales voted to allow women to become bishops at a vote in 2013.