Welwyn Garden City (/ˈwɛlɪn/ ^ⓘ WEL-in) is a town in Hertfordshire, England, 20 miles (32 km) north of London. It was the second garden city in England (founded 1920) and one of the first new towns (designated 1948). It is unique in being both a garden city and a new town and exemplifies the physical, social and cultural planning ideals of the periods in which it was built.

Welwyn Garden City was founded by Sir Ebenezer Howard in 1920 following his previous experiment in Letchworth Garden City. Howard had called for the creation of planned towns that were to combine the benefits of the city and the countryside and to avoid the disadvantages of both. It was designed to be 'The Perfect Town'. The Garden Cities and Town Planning Association had defined a garden city as

a town designed for healthy living and industry of a size that makes possible a full measure of social life but not larger, surrounded by a rural belt; the whole of the land being in public ownership, or held in trust for the community

In 1919, Howard arranged for the purchase of land in Hertfordshire that had already been identified as a suitable site. A company called Second Garden City Limited was formed in October 1919 to start buying the land and developing the town. On 4 February 1920, the company's board decided to call the new garden city Digswell, taking the name of the existing small village which would be surrounded by the development. Six days later they changed their minds, deciding instead to call it Welwyn Garden City, reflecting that the project was already being discussed generally as the "new garden city near Welwyn". On 29 April 1920, the company changed its name to become Welwyn Garden City Limited. Sir Theodore Chambers chaired the company, whilst Louis de Soissons was appointed as architect and town planner, Charles Purdom as finance director and Frederic Osborn as secretary. The first house was occupied just before Christmas 1920.

The town is laid out along tree-lined boulevards with a neo-Georgian town centre. It has its own environmental protection legislation, the Scheme of Management for Welwyn Garden City. Every road has a wide grass verge. The spine of the town is Parkway, a central mall or scenic parkway, almost a mile long. The view along Parkway to the south was once described as one of the world's finest urban vistas. Older houses are on the west side of Parkway and newer houses on the east side

The original planners intended that all the residents of the garden city would shop in one shop and created the Welwyn Stores, a monopoly which caused some local resentment. Commercial pressures have since ensured much more competition and variety, and the Welwyn Stores were in 1984 taken over by the John Lewis Partnership.

In 1948, Welwyn Garden City was designated a new town under the New Towns Act 1946 (9 & 10 Geo. 6. c. 68) and the Welwyn Garden City company handed its assets to the Welwyn Garden City Development Corporation. Louis de Soissons remained as its planning consultant. That year The Times compared Welwyn Garden City with Hatfield. It described Welwyn Garden City as a world-famous modern new town developed as an experiment in community planning and Hatfield as an unplanned settlement created by sporadic building in the open country. "Welwyn, though far from perfect, made the New Towns Act possible, just as Hatfield, by its imperfection, made it necessary." In 1966, the Development Corporation was wound up and handed over to the Commission for New Towns. The housing stock, neighbourhood shopping and green spaces were passed to Welwyn Hatfield District Council between 1978 and 1983.

The New QEII Hospital, completed in June 2015, offers outpatient, diagnostic and ante/postnatal services.