The twentieth bishop was Wulfsige III (or St. Wulfsin). In 998 he established a Benedictine abbey at Sherborne and became its first abbot. In 1075 the bishopric of Sherborne was transferred to Old Sarum, so Sherborne remained an abbey church but was no longer a cathedral. The bishop (in Old Sarum) remained the nominal head of the abbey until 1122, when Roger de Caen, Bishop of Salisbury, made the abbey independent.
The abbey was rebuilt in the 12th century, in Norman style, and again in the 15th century, in Perpendicular style. The fan-vaulting in the choir for which Sherborne is still famous was added in that 15th century remodeling by Abbot John Brunyng (1415-1436).
The Benedictine foundation at Sherborne ended in the Dissolution of the Monasteries in 1539, but instead of surrendering the abbey to King Henry VIII, the people of Sherborne (as the people of many other places did) bought the building to be their parish church, which it still is. In 1550 King Edward VI issued a new charter to the school that had existed at Sherborne since 705, and some of the remaining abbey buildings were turned over to it.