The Diocese of Plymouth(Latin: Dioecesis Plymuthensis) is a Latin Church diocese of the Catholic Church in England. The episcopal see is in the city of Plymouth, Devon, where the bishop's seat (cathedra) is located at the Cathedral Church of St Mary and St Boniface.

The Cathedral Church of Saint Mary and Saint Boniface in Plymouth, England, is the seat of the Bishop of Plymouth and mother church of the Roman Catholic Diocese of Plymouth, which covers the counties of Cornwall, Devon and Dorset. The Diocese of Plymouth was created in 1850 after the issuing of the papal bull Universalis Ecclesiae. In 1858 the new condign cathedral was opened and put under the patronage of the Virgin Mary and Saint Boniface, the latter thought to have been born in Crediton in the area of the diocese.