10 Downing Street in London is the official residence and office of the prime minister of the United Kingdom.

Colloquially known as Number 10, the building is in Downing Street in the City of Westminster, London. It is over 300 years old and contains approximately 100 rooms. A private residence for the prime minister occupies the third floor and there is a kitchen in the basement. The other floors contain offices and conference, reception, sitting and dining rooms where the prime minister works, and where government ministers, national leaders and foreign dignitaries are met and hosted. At the rear is an interior courtyard and a terrace overlooking a 1⁄2 acre (0.2 ha) garden. Adjacent to St James's Park, Number 10 is approximately 3⁄4 mile (1.2 km) from Buckingham Palace, the London residence of the British monarch, and near the Palace of Westminster, the meeting place of both Houses of Parliament.

Originally three houses, Number 10 was offered to Robert Walpole by King George II in 1732. Walpole accepted on the condition that the gift was to the office of First Lord of the Treasury. The post of First Lord of the Treasury has, for much of the 18th and 19th centuries and invariably since 1905, been held by the prime minister. Walpole commissioned William Kent to join the three houses and it is this larger house that is known as Number 10 Downing Street.

Despite its size and convenient location near to Parliament, few early prime ministers lived at 10 Downing Street. Costly to maintain, neglected, and run-down, Number 10 was scheduled to be demolished several times, but the property survived and became linked with many statesmen and events in British history. In 1985, Margaret Thatcher said Number 10 had become "one of the most precious jewels in the national heritage".

10 Downing Street is Government property. Its registered legal title is held in the name of the secretary of state for levelling up, housing and communities (the secretary of state is a corporation sole).