Thatcherism is a form of British conservative ideology named after Conservative Party leader Margaret Thatcher that relates to not just her political platform and particular policies but also her personal character and style of management while in office. Proponents of Thatcherism are referred to as Thatcherites. The term has been used to describe the principles of the British government under Thatcher from the 1979 general election to her resignation in 1990. In international terms, Thatcherites have been described as a part of the general socio-economic movement known as neoliberalism, with different countries besides the United Kingdom (such as the United States) sharing similar policies around expansionary capitalism.

Thatcherism represents a systematic, decisive rejection and reversal of the post-war consensus inside Great Britain in terms of governance, whereby the major political parties largely agreed on the central themes of Keynesianism, the welfare state, nationalised industry, and close regulation of the British economy before Thatcher's rise to prominence. Under her administration, there was one major exception to Thatcherite changes: the National Health Service (NHS), which was widely popular with the British public. In 1982, Thatcher promised that the NHS was "safe in our hands".

The exact terms of what makes up Thatcherism and its specific legacy in British history over the past decades are controversial. Ideologically, Thatcherism has been described by Nigel Lawson, Thatcher's Chancellor of the Exchequer from 1983 to 1989, as a political platform emphasising free markets with restrained government spending and tax cuts that gets coupled with British nationalism both at home and abroad. Thatcher herself rarely used the word "Thatcherism". However, she gave a speech in Solihull during her campaign for the 1987 general election and included in a discussion of the economic successes the remark: "that's what I call Thatcherism".

The Daily Telegraph stated in April 2008 that the programme of the next non-Conservative government, with Tony Blair's "New Labour" organisation governing the nation throughout the 1990s and 2000s, basically accepted the central reform measures of Thatcherism such as deregulation, privatisation of key national industries, maintaining a flexible labour market, marginalising the trade unions and centralising power from local authorities to central government. While Blair distanced himself from certain aspects of Thatcherism earlier in his career, in his 2010 autobiography A Journey, he argued both that "Britain needed the industrial and economic reforms of the Thatcher period" and as well that "much of what she wanted to do in the 1980s was inevitable, a consequence not of ideology but of social and economic change."

[A] mixture of free markets, financial discipline, firm control over public expenditure, tax cuts, nationalism, "Victorian values" (of the Samuel Smiles self-help variety), privatisation and a dash of populism.

— Nigel Lawson's definition of Thatcherism

Thatcherism attempts to promote low inflation, the small state and free markets through tight control of the money supply, privatisation and constraints on the labour movement. It is often compared with Reaganomics in the United States, economic rationalism in Australia and Rogernomics in New Zealand and as a key part of the worldwide economic liberal movement.

Thatcherism is thus often compared to classical liberalism. Milton Friedman said that "Margaret Thatcher is not in terms of belief a Tory. She is a nineteenth-century Liberal". Thatcher herself stated during a speech in 1983: "I would not mind betting that if Mr Gladstone were alive today he would apply to join the Conservative Party". In the 1996 Keith Joseph memorial lecture, Thatcher argued: "The kind of Conservatism which he and I [...] favoured would be best described as 'liberal', in the old-fashioned sense. And I mean the liberalism of Mr Gladstone, not of the latter day collectivists". Thatcher once told Friedrich Hayek: "I know you want me to become a Whig; no, I am a Tory". Hayek believed "she has felt this very clearly". The relationship between Thatcherism and liberalism is complicated. Thatcher's former defence secretary John Nott claimed that "it is a complete misreading of her beliefs to depict her as a nineteenth-century Liberal".