The chancellor of the exchequer, often abbreviated to chancellor, is a senior minister of the Crown within the Government of the United Kingdom, and the head of His Majesty's Treasury. As one of the four Great Offices of State, the chancellor is a high-ranking member of the British Cabinet.

Responsible for all economic and financial matters, the role is equivalent to that of a finance minister in other countries. The chancellor is now always second lord of the Treasury as one of at least six lords commissioners of the Treasury, responsible for executing the office of the Treasurer of the Exchequer – the others are the prime minister and Commons government whips. In the 18th and early 19th centuries, it was common for the prime minister also to serve as Chancellor of the Exchequer if he sat in the Commons; the last Chancellor who was simultaneously prime minister and Chancellor of the Exchequer was Stanley Baldwin in 1923. Formerly, in cases when the chancellorship was vacant, the lord chief justice of the King's Bench would act as chancellor pro tempore. The last lord chief justice to serve in this way was Lord Denman in 1834.

The chancellor is the third-oldest major state office in English and British history, and in recent times has come to be the most powerful office in British politics after the prime minister. It originally carried responsibility for the Exchequer, the medieval English institution for the collection and auditing of royal revenues. The earliest surviving records which are the results of the exchequer's audit date from 1129 to 1130 under King Henry I and show continuity from previous years. The chancellor has oversight of fiscal policy, therefore of taxation and public spending across government departments. It previously controlled monetary policy as well until 1997, when the Bank of England was granted independent control of its interest rates.

Since 1718, all chancellors of the exchequer, except at times the lord chief justice as interim holders, have been members of the House of Commons, with Lord Stanhope being the last chancellor from the House of Lords.

The office holder works alongside the other Treasury ministers and the permanent secretary to the Treasury. The corresponding shadow minister is the shadow chancellor of the Exchequer, and the chancellor is also scrutinised by the Liberal Democrat Treasury spokesperson and the Treasury Select Committee.

The current chancellor is Rachel Reeves.

The holder of the office of chancellor of the exchequer is ex officio second lord of the Treasury as a member of the commission exercising the ancient office of treasurer of the exchequer. As second lord, her official residence is 11 Downing Street in London, next door to the residence of the first lord of the Treasury (a title that has for many years been held by the prime minister), who resides in 10 Downing Street. While in the past both houses were private residences, today they serve as interlinked offices, with the occupant living in an apartment made from attic rooms previously resided in by servants.

Since 1827, the chancellor has always simultaneously held the office of second lord of the Treasury when that person has not also been the prime minister.