The Roman Catholic Diocese of Portsmouth (Latin: Dioecesis Portus Magni)is a Latin diocese of the Catholic Church that covers the Channel Islands as well as parts of England (Hampshire, the Isle of Wight and parts of Berkshire, Dorset and Oxfordshire).

The diocese is part of the metropolitan Province of Southwark, which covers all of the far South of England as well as the Channel Islands.

The Diocese of Portsmouth, situated centrally within the Metropolitan Province of Southwark, extending as far as Abingdon in the North; and down to and including the Channel Islands in the South, and roughly from Liphook in the East to Andover in the West.

[1] The Southwark diocese had become too large for one bishop, extending as it did from London to Bournemouth and from the outskirts of Oxford to Dover and including the Channel Islands.

The construction of a large parish church in the centre of Portsmouth had begun, and it was decided to make it the future cathedral of the diocese.

He was consecrated by Cardinal Manning on 25 July 1882 and on 10 August of that year opened the Cathedral of St John the Evangelist at Portsmouth.

[1] Since 1900 the diocese has had seven bishops: John Cahill, 1900–1910; William Cotter (who came from Cloyne in Co. Cork), 1910–1940; John Henry King (who was given the personal title of archbishop in 1954), 1941–1965; Derek Worlock (translated to Liverpool as Archbishop 7 February 1976), 1965–1976; Anthony Emery, 1976–1988; Crispian Hollis, 1988–2012;[2] Philip Egan, 2012-.

[4] Bishop Egan's Episcopal Ordination took place at St John's Cathedral, Portsmouth, on Monday 24 September 2012, the Feast of Our Lady of Walsingham.

Roger Francis Crispian Hollis, bishop emeritus of Portsmouth, was educated at Stonyhurst College and ordained on 11 July 1965.

[4] Until 2006 the diocese was organised into the deaneries of Portsmouth, Aldershot, Alton, Basingstoke, Bournemouth, Fareham, Havant, New Forest, Southampton, Winchester, North East Berks, South Berks, Kennet Valley, Vale of the White Horse, Reading, Isle of Wight, Guernsey and Jersey.

Following a period of consultation, the deaneries were replaced with 24 new pastoral areas at Pentecost 2006, with the aim to eventually turn them into parishes in the future.