Eaton Square is a residential garden square in London's Belgravia district. It is one of the three garden squares built by the Grosvenor family when they developed the main part of Belgravia in the 19th century, and is named after Eaton Hall, the Grosvenor country house in Cheshire. Eaton Square is larger but less grand than the central feature of the district, Belgrave Square, and both larger and grander than Chester Square. The first block was laid out by Thomas Cubitt from 1827.
The houses in Eaton Square are large, predominantly three-bay-wide buildings, joined in regular terraces in a classical style, with four or five main storeys, plus attic and basement and a mews house behind. The square is one of London's largest and is divided into six compartments by the upper end of Kings Road (northeast of Sloane Square), a main road, now busy with traffic, that occupies its long axis, and two smaller cross streets. Most of the houses are faced with white stucco, but some are faced with brick.
Before World War II Eaton Square was a securely upper class address, but not of the grandeur of London's very grandest addresses in Mayfair and Belgravia: Belgrave Square, Grosvenor Square, St James's Square or Park Lane.
However, after World War II, when those places were converted to mainly commercial and institutional use, Eaton Square remained almost wholly residential and rose to the front rank of fashionable addresses. Some of the houses remain undivided, but much of the square has been converted into flats and maisonettes by the Grosvenor Estate. These are often lateral conversions - that is, they cut across more than one of the original houses - and they usually cost several million pounds. The exterior appearance of the square remains as it was when it was built, with no intrusive modern buildings. Most but not all of the freeholds still belong to the Grosvenor Group, and the present Duke of Westminster has his own London home in the square - an illustration of the migrations of the London elite already mentioned, as until the 1920s his predecessors lived in a mansion on the site of the present Grosvenor House Hotel in Park Lane.
At the east end of the square is St Peter's, a large Church of England church, in a classical style, which features a six-columned Ionic portico and a clock tower. It was designed by Henry Hakewill and built between 1824 and 1827 during the first development of Eaton Square.
Between 1940 and 1944 the Belgian government in exile was located in Eaton Square.
Eaton Square is the address of Prince Amerigo and his wife, the former Maggie Verver, in the last complete major novel by Henry James, The Golden Bowl. In the original newspaper piece that was expanded into Gilbert and Sullivan's Trial by Jury, the judge invites the rest of the cast to his house in "Five hundred and eleven, Eaton Square" for the wedding breakfast. In Angela Carter's last novel, Wise Children, Eaton Square is visited by Peregrine Hazard after returning by cab from the beach. In Anthony Trollope's novel The Bertrams Sir Henry Harcourt and his unhappy bride Lady Harcourt (Caroline Waddington) take a house in Eaton Square after their marriage. In Jeffrey Archer's First Among Equals, the Hon. Charles Gurney Seymour, future cabinet minister and son of the Earl of Bridgwater, and his wife Lady Fiona, daughter of the Duke of Falkirk, live in Eaton Square.
In the series Downton Abbey the character of Lady Rosamund Painswick, sister of Robert Crawley, Earl of Grantham, maintains an address in Eaton Square.
- No. 1: Lord Boothby - parliamentarian and political commentator
- No. 7: Sean Connery - Actor
- No. 36: Ruth Roche, Baroness Fermoy - long-time confidante of Queen Elizabeth, the Queen Mother, and maternal grandmother of Diana, the Princess of Wales
- No. 37: Neville Chamberlain - British Prime Minister
- No. 37: Joachim von Ribbentrop - German Ambassador to London
- No. 42: Peter Thorneycroft - British Chancellor of the Exchequer
- No. 44: Prince Metternich - Austrian statesman
- No. 45: George Tryon - British Admiral who died in sinking of the HMS Victoria in 1893
- No. 54: Vivien Leigh, Oscar-winning actress; Luise Rainer, Oscar-winning actress from 1936 and 1937
- No. 72: Sir Robert Helpmann - Actor, Dancer and choreographer, mostly remembered for his role in the film Chitty Chitty Bang Bang.
- No. 75: Rex Harrison - Oscar-winning actor
- No. 80: George Peabody - American banker and philanthropist
- No. 82: Queen Wilhelmina of the Netherlands in 1940. 
- No. 86: Lord Halifax - British Foreign Secretary
- No. 93: Stanley Baldwin - British Prime Minister
- No. 97: Sir Francis Scott Bt and Lady Scott of Great Barr (d. 1863 and 1909 respectively)
- No. 100: Gerald Grosvenor, 6th Duke of Westminster - the freeholder of most of the square and most of the surrounding district.
- No. 112: Leo Amery, politician and minister in Churchill's wartime cabinet; and his son Julian Amery, Baron Amery of Lustleigh, Conservative MP.
- No. 114: Harry Stuart Goodhart-Rendell, Architect
- No. 106: The embassy of Bolivia
- No. 118: Sir William Corry, Bart., of Dunraven, Co. Antrim (d. 1926)
- No. 57 Lower Belgrave St (corner of Eaton Sq.) Roman Abramovich - Russian billionaire and the main owner of Chelsea Football Club
- George Soros - Hungarian-born hedge fund manager. 
- Henry Hughes Wilson - Field Marshal
- Princess Katherine of Greece and Denmark
- Alfred Robens, Baron Robens of Woldingham - British politician and chairman of the National Coal Board
- Sir James Monro - Commissioner Metropolitan Police
- Kabaka Sir Fredrick Edward Mutesa, the first president of Uganda, lived at no. 12 Eaton Place.
- Mr. & Mrs. Ernest Aldrich Simpson from 1958
- Gilbert, W. S.; Sullivan, Arthur (2001). Ian Bradley, ed. The Complete Annotated Gilbert and Sullivan. Oxford: Oxford University Press. p. 36.
- Open Plaques - Lord Boothby, 1 Eaton Sq
- Eaton Mess for London's finest Square
- Who's Who, 1980 (Adam and Charles Black, London) p. 837
- Open Plaques - Neville Chamberlain, 37 Eaton Sq
- Open Plaques - Prince Metternich, 44 Eaton Sq
- Open Plaques - Vivien Leigh, 54 Eaton Sq
- Open Plaques - George Peabody, 93 Eaton Sq
- H.M. (koningin Wilhelmina) koningin Wilhelmina Helena Pauline Maria, koningin der Nederlanden, prinses van Oranje-Nassau
- Open Plaques - Edward Wood 1st Earl of Halifax, 86 Eaton Sq
- Open Plaques - Stanley Baldwin, 93 Eaton Sq
- Daily Mail - Abramovich 'gives ex-wife £1bn and four homes in world's costliest divorce'
- Rich pickings for the hawk of Eaton Square