Adelphi (Greek: adelphoi, "brothers") is a district of the City of Westminster in London.<ref name=mills>Mills, A., Oxford Dictionary of London Place Names, (2001)</ref> The small district includes the streets of Adelphi Terrace, Robert Street and John Adam Street.<ref name=mills/>
The district is named after the Adelphi Buildings, a block of 24 unified neoclassical terrace houses occupying the land between The Strand and the River Thames in the parish of St Martin in the Fields. They were built between 1768–72, by the Adam brothers (John, Robert, James and William Adam). The ruins of Durham House on the site were demolished for their construction. The nearby Adelphi Theatre is named after the Adelphi Buildings. Robert Adam was influenced by his extensive visit to Diocletian's Palace in Dalmatia, and applied some of this influence to the design of the neoclassical Adelphi Buildings. The Adelphi Buildings were demolished in the early 1930s and replaced with the New Adelphi, a monumental Art Deco building designed by the firm of Collcutt & Hamp. Only one building from the old Adelphi remains, situated at 11 Adelphi Terrace and occupied by numismatic specialists A.H. Baldwin & Sons Ltd.
- Edward Litt Laman Blanchard, writer, lived in Adelphi Terrace from 1876 to 1889
- David Garrick lived for his final seven years, and died in 1779, in the centre house of the buildings.
- Thomas Monro, Physician to George III and art patron, owned a house in Adelphi Terrace.
- Richard D'Oyly Carte, Victorian impresario
- Sir J M Barrie (1860–1937), playwright and novelist, author of Peter Pan, at Adelphi Terrace
- John Galsworthy, novelist, author of The Forsyte Saga
- George Bernard Shaw, Irish playwright, Fabian socialist, co-founder of the London School of Economics and Political Science
- Thomas Hardy, English novelist
- Charles Booth, Shipyard owner, Philanthropist, coordinator and co-author of the monumental Life and Labour of the People in London (1880–1900).
- Fictional detective Gideon Fell, created by John Dickson Carr, lived at no. 1, Adelphi Terrace.
- David Copperfield, created by Charles Dickens, lived in Adelphi.
The Adelphi building was used for some scenes in ITV's Agatha Christie's Poirot episode The Theft of the Royal Ruby .
- Brereton, Austin. The literary history of the Adelphi and its neighbourhood (New York: Duffield, 1909). Illustrated.
- List of demolished buildings and structures in London