The Diocese of Westminster was a short-lived diocese of the Church of England, extant from 1540 - 1550.
The Diocese was created from part of the Diocese of London, and comprised Westminster (which was raised to the dignity of a city), and the county of Middlesex, with the exception of Fulham. Fulham was excluded as it was the site of Fulham Palace, residence of the Bishop of London.
Ten years later in 1550, on Thirlby's move to become Bishop of Norwich, the diocese was merged back into the Diocese of London. Westminster retained its city status. The cathedral became an abbey again six years later.