John Nash (1752 – 13 May 1835) was a British architect responsible for much of the layout of Regency London.

Born in London as the son of a Lambeth millwright, Nash trained with architect Sir Robert Taylor, but his own career was initially unsuccessful and short-lived. After inheriting a substantial fortune, he retired to live in Wales, but he lost much of his fortune through bad investments and was declared bankrupt in 1783. This forced him to resume work as an architect, focusing initially on the design of country houses, in a successful partnership with landscape garden designer, Humphry Repton; the pair would collaborate to carefully place the Nash-designed building in grounds designed by Repton. Eventually, Nash felt able to return to work in London, in 1792.

Nash's work came to the attention of the Prince Regent (later King George IV) who, in 1811 commissioned him to develop an area then known as Marylebone Park. With the Regent's backing (and major inputs from Repton), Nash created a master plan for the area, put into action from 1818 onwards, which stretched from St James’s northwards and included Regent Street, Regent's Park and its neighbouring streets, terraces and crescents of elegant town houses and villas. Nash did not complete all the detailed designs himself; in some instances, completion was left in the hands of other architects such as James Pennethorne and the young Decimus Burton.

Nash was also a director of the Regent's Canal Company set up in 1812 to provide a canal link from west London to the River Thames in the east. Nash's masterplan provided for the canal to run around the northern edge of Regent's Park; as with other projects, he left its execution to one of his assistants, in this case James Morgan. The first phase of the Regent's Canal opened in 1816.

Further London commissions for Nash followed, including the remodelling of Buckingham House to create Buckingham Palace (1825-1835), plus the Royal Mews and Marble Arch (originally designed as a triumphal arch to stand at the end of the Mall, it was found to be too narrow for the royal State Coach and was moved in 1851 to its current location at the western end of Oxford Street).

Other London projects included:

• Trafalgar Square
• St. James's Park
• Haymarket Theatre (1820)
• the Church of All Souls, Langham Place (1822-25)
• Carlton House Terrace (1827-1833)
• Cumberland Terrace (1827)

Outside London, his work included:

• rebuilding of the Royal Pavilion at Brighton (1815-1822)
• East Cowes Castle on the Isle of Wight – his home for many years
• Blaise Hamlet, Bristol
• Grovelands Park, Enfield, Middlesex

He died and was buried at St James Church, Cowes.

In modern times, some researchers have speculated that Nash may have been a high-functioning Autist, a.k.a. a person with Asperger's Syndrome, based on descriptions of his personality.