The Roman Catholic Diocese of Westminster is an archdiocese^[1] of the Latin Rite of the Roman Catholic Church in England. The diocese consists of all of London north of the River Thames and west of the River Lea, the borough of Spelthorne (in Surrey), and the county of Hertfordshire, which lies immediately to London's north.

The diocese is led by the Archbishop of Westminster, who serves as pastor of the mother church, Westminster Cathedral, as well as the metropolitan bishopof the ecclesiastical Province of Westminster. Since the re-establishment of the English Catholic dioceses in 1850, each Archbishop of Westminster—including the incumbent, Cardinal Vincent Gerard Nichols—has been created a cardinalby the Pope in consistory, often as the only cardinal in England, but the 43rd of English cardinals since the 12th century. It is also customary for the Archbishop of Westminster to be elected President of the Catholic Bishops' Conference of England and Wales providing a degree of a formal direction for the other English bishops and archbishops. However, he is not formally a primate, though he has special privileges conferred by the Papal Bull Si qua est.^[2] The Archbishop of Westminster has not been granted the title of Primate of England and Wales, which is sometimes applied to him, but his position has been described as that of "chief metropolitan" of the Roman Catholic Church in England and Wales and as "similar to" that of the Archbishop of Canterburyin the Church of England (as the metropolitan bishop of the Diocese of Canterbury.^[3] The diocese is one of the smallest dioceses in England and Wales in geographical area, but the largest in terms of Catholic population and priests.^[4]

The suffragan sees of Westminster are the dioceses of Brentwood, East Anglia, Northampton, and Nottingham.