Duke of Westminster (Herzog von Westminster) ist ein erblicher britischer Adelstitel in der Peerage of the United Kingdom. Der Titel ist nach dem Londoner Stadtteil benannt.

Der hauptsächlich genutzte Landsitz der Familie ist Eaton Hall, in der Nähe von Chester in Cheshire; daneben wird Ely Lodge im nordirischen County Fermanagh genutzt. Die Familie besaß in der Vergangenheit außerdem ein großes Stadthaus in der Londoner Park Lane, das Grosvenor House genannt wurde.

Der Titel wurde am 27. Februar 1874 von Königin Victoria an Hugh Grosvenor, 3. Marquess of Westminster, verliehen. Er führte bereits seit 1869 die fortan nachgeordneten Titel 3. Marquess of Westminster, 4. Earl Grosvenor, 4. Viscount Belgrave, 4. Baron Grosvenor und 11. Baronet, of Eaton.

Der Titel Marquess of Westminster war am 13. September 1831 in der Peerage of the United Kingdom seinem Großvater Robert Grosvenor, 2. Earl Grosvenor, bei der Krönung von Wilhelm IV. verliehen worden. Dessen Vater Sir Richard Grosvenor, 7. Baronet, war von Georg III. in der Peerage of Great Britain am 8. April 1761 zum Baron Grosvenor, of Eaton in the County of Chester, und am 5. Juli 1784 zum Earl Grosvenor und Viscount Belgrave, of Belgrave in the County of Chester, erhoben worden. Dessen Ur-ur-urgroßvater, der Unterhausabgeordnete Sir Richard Grosvenor, war am 23. Februar 1622 in der Baronetage of England der Titel Baronet, of Eaton in the County of Chester, verliehen worden.

Der Heir Apparent des jeweiligen Herzogs führt den Höflichkeitstitel Earl Grosvenor, dessen Heir Apparent den Höflichkeitstitel Viscount Belgrave.

Der heutige Duke of Westminster zählt mit einem geschätzten Vermögen von ca. 9,9 Mrd. £ zu den reichsten Männern Großbritanniens. Im Forbes Magazine von 2009 wurde er in der Liste der reichsten Personen der Welt auf Platz 29 geführt. Er verfügt u. a. über Grundbesitz von ca. 1,2 km2 Größe in den Londoner Nobelstadtteilen Mayfair, Belgravia und Pimlico, davon allein 770.000 m2 im an den Buckingham Palast angrenzenden Belgravia, sowie über den Landsitz Eaton Hall in Cheshire.

In 1999, Lord Portman became the 10th Viscount and began work on rejuvenating the Estate.

Over the last 20 years, the development of the Estate has gone from strength to strength. We actively manage the property assets, both directly as a landlord to an ever-increasing number of residential and commercial customers, and through our relationships with long-term leaseholders.

We aim to be at the forefront of

property management and placemaking in Central London. This is an example of how an estate can combine a modern, agile and forward-thinking approach to its activities, while remaining loyal to its history, heritage and values.

The Portman Estate dates back to the 16th century, when Sir William Portman, Lord Chief Justice to King Henry VIII,^[5] and originally from Orchard Portman in Somerset, leased 270 acres of the Manor of Lileston (Lisson). He acquired the freehold in 1554, but most of the land remained farmland and meadow until the mid-18th century and the building boom after the end of the Seven Years' War in 1763.

In the 1750s William Baker had leased land from the family to lay out Orchard and Portman Streets, and the north side of Oxford Street. Henry William Portman, a descendant of Sir William, continued the development in 1764 with the creation of Portman Square, with buildings by James Wyatt, Robert Adam and James 'Athenian' Stuart, including Montagu House, built in the north-west corner for the famed literary hostess Elizabeth Montagu and later used by the Portman family as their London town house.

Portman Square was the focus of the new estate and was followed by the building of Manchester Square during the 1770s and Bryanston and Montagu Squares 30 years later. These were laid out by the Estate's architect, James Thompson Parkinson. The area remained largely residential, attracting the prosperous middle class who wanted to live near the centre of London. There were also mews for tradesmen and servants. At the southwest corner of the Estate, where Marble Arch now stands, was the Tyburn gallows, London's principal place of public execution until 1783.

Development of the area north of the Marylebone Road around Dorset Square continued after 1815, and to the North West in Lisson Green, workers’ cottages were built from 1820 to 1840. Many of the original Georgian houses north of Portman Square were redeveloped as mansion blocks, which were let on long leases. This development spread along the major traffic routes of Edgware Road and Baker Street.

In 1948 the Estate, then valued at £10 million, was subject to death duties of £7.6 million on the death of the seventh Viscount Portman, resulting in the sale of all the family's West Country estates as well as the northern part of the London Estate in 1951, and the area around Crawford Street the following year. In the later 1950s and 1960s the Estate collaborated with the developer Max Rayne to redevelop the frontage of Oxford Street and Baker Street, as well as the south and west sides of Portman Square.