The Greater London Authority (GLA), colloquially known by the metonym City Hall, is the devolved regional governance body of Greater London, England. It consists of two political branches: an executive Mayor (currently Sadiq Khan) and the 25-member London Assembly, which serves as a means of checks and balances on the Mayor. The authority was established in 2000, following a local referendum, and derives most of its powers from the Greater London Authority Act 1999 and the Greater London Authority Act 2007.

It is a strategic regional authority, with powers over transport, policing, economic development, and fire and emergency planning. Three functional bodies —Transport for London, the Mayor's Office for Policing and Crime, and the London Fire Commissioner— are responsible for delivery of services in these areas. The planning policies of the Mayor of London are detailed in a statutory London Plan which is regularly updated and published.

The Greater London Authority is mostly funded by direct government grant and it is also a precepting authority, with some money collected with local Council Tax. The GLA is unique in the British devolved and local government system, in terms of structure (it uses a presidential system-esque model), elections and selection of powers. The authority was established to replace a range of joint boards and quangos and provided an elected upper tier of local government in Greater London for the first time since the abolition of the Greater London Council in 1986.

Since May 2016, both branches have been under the control of the London Labour Party.

The GLA is responsible for the strategic administration of the 1,579 km² (610 sq mi) of Greater London. It shares local government powers with the councils of 32 London boroughs and the City of London Corporation. It was created to improve the co-ordination between the local authorities in Greater London, while the Mayor of London's role is to provide a single representative for the capital. The Mayor proposes policy and the GLA's budget, and makes appointments to the capital's strategic executive such as Transport for London.

The London Assembly serves as a watchdog for the city and holds the mayor accountable. The assembly must also accept or amend the mayor's budget on an annual basis. The GLA is based at City Hall in the London Borough of Newham, situated next to the redeveloped Royal Victoria Dock in Canning Town. The GLA moved to this building from the previous City Hall, in Southwark, in January 2022.

The GLA is different from the corporation of the City of London with its largely ceremonial lord mayors, which controls only the square mile of the city, London's chief financial centre.

In 1986, the Greater London Council was abolished by the Conservative government of Margaret Thatcher. Many people have surmised that the decision to abolish the GLC was made because of the existence of a high-spending left-wing Labour administration under Ken Livingstone, although pressure for the abolition of the GLC had arisen before Livingstone took over, and was largely driven by the belief among the outer London borough councils that they could perform the functions of the GLC just as well.