The Bishop of Leighlin was a separate episcopal title which took its name after the small town of Old Leighlin in County Carlow, Republic of Ireland.
The title is now united with other bishoprics. In the Church of Ireland, it is held by the Bishop of Cashel and Ossory, whose full title is the Bishop of Cashel, Waterford, Lismore, Ossory, Ferns and Leighlin. In the Roman Catholic Church, it is held by the Bishop of Kildare and Leighlin.
The diocese of Leighlin was one of the twenty-four dioceses established at the Synod of Rathbreasail in 1111. Following the Reformation, there are parallel apostolic successions. In the Church of Ireland, Leighlin was combined with Ferns in 1597 to form the united bishopric of Ferns and Leighlin. In the Roman Catholic Church, the see was governed by bishops or vicars apostolic, and from 1678 to 1694 it was administered by the Bishops of Kildare. The formal union of Kildare and Leighlin was decreed on 29 November 1694, thereby forming the united Diocese of Kildare and Leighlin.