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.

Erected as the Diocese of Plymouth in 1850 by Pope Pius IX, from the Apostolic Vicariate of the Western District, the diocese has remained jurisdictionally constant since. Since 1965, the diocese has been a suffragan see of the Ecclesiastical Province of Southwark; before then, from 1850 to 1911 it was in the Province of Westminster, then from 1911 to 1965 in the Province of Birmingham.

In December 2023, Christopher Whitehead from the Diocese of Clifton was appointed bishop-elect of the diocese, replacing Bishop Mark O’Toole, who had been appointed as Archbishop of Cardiff a year prior. His episcopal ordination was due to take place on the 22nd February. On 1st February, the Catholic Bishops' Conference of England and Wales released a statement that Whitehead was under canonical investigation, and that the ordination would not take place, no reason was given. The diocese remains under a diocesan administrator.

The diocese covers the counties of Cornwall, Devon and Dorset, stretching from Penzance and the Isles of Scilly in the west, to parts of Bournemouth in the east. It is divided into five deaneries: Cornwall, Dorset, Exeter, Plymouth, and Torbay. There are chaplaincies at the universities of Bournemouth, Exeter and Plymouth.

The diocese includes the Grail Centre in Pinner in the London Borough of Harrow (which is physically in the Diocese of Westminster), a lay community of single Roman Catholic women. The Centre promotes a wider "Grail community" to include non-resident women and families, and also publishes a translation of the Psalms.

• George Errington (Appointed on 27 June 1851 – Translated to Westminster as coadjutor archbishop on 30 March 1855)
• William Vaughan (Appointed on 10 July 1855 – Died on 24 October 1902)
• Charles Maurice Graham (Succeeded on 25 October 1902 – Retired on 16 March 1911)
• John Joseph Keily (Appointed on 21 April 1911 – Died on 23 September 1928)
• John Patrick Barrett (Appointed on 7 June 1929 – Died on 2 November 1946)
• Francis Joseph Grimshaw (Appointed on 2 June 1947 – Translated to Birmingham as metropolitan archbishop on 11 May 1954)
• Cyril Edward Restieaux (Appointed on 9 April 1955 – Retired on 19 November 1985)
• Hugh Christopher Budd (Appointed on 19 November 1985 – Retired on 9 November 2013)
• Mark O'Toole (Appointed on 9 November 2013 – Translated to Cardiff as metropolitan archbishop and Menevia on 27 April 2022)

• Charles Maurice Graham (1891-1902)
• James Moor (1890), did not take effect

• Robert Brindle, appointed auxiliary bishop of Westminster in 1899, and then Bishop of Nottingham in 1901.
• Robert Bernard Brownlow, appointed Bishop of Clifton in 1894

Cornwall: Bodmin (SS Mary & St Petroc), Tintagel (St Paul the Apostle), Falmouth (St Mary's)

Devon: Exeter Sacred Heart, Plymouth Cathedral of St Mary and St Boniface, Torquay (Assumption of Our Lady) and Torquay (Our Lady Help of Christians and St Denis)

Dorset: Dorchester (Holy Trinity), Weymouth (St Joseph)

Monasteries, abbeys and priories: Buckfast Abbey, Ivybridge St Austin's Priory, Lanherne Carmelite Community, Sclerder Abbey

• Catholic Church in England and Wales
• List of Catholic churches in the United Kingdom

• "Diocese of Plymouth". Catholic-Hierarchy.org. David M. Cheney.
• Roman Catholic Diocese of Plymouth website
• Plymouth Cathedral
• Grail Centre website
• GCatholic.org
• Herbermann, Charles, ed. (1913). "Diocese of Plymouth" . Catholic Encyclopedia. New York: Robert Appleton Company.