Peter Benjamin Mandelson, Baron Mandelson, (পিটার বেঞ্জামিন ম্যান্ডেলসন, ব্যারন ম্যান্ডেলসন) is a British former Labour Party politician, lobbyist and diplomat. He was Member of Parliament (MP) for Hartlepool from 1992 to 2004 and served in the Cabinets of Tony Blair and Gordon Brown as Minister without portfolio, Secretary of State for Northern Ireland, Business Secretary and First Secretary of State. He was British Ambassador to the United States in 2025 under Prime Minister Sir Keir Starmer.

A member of the Labour Party throughout his career, Mandelson served as its director of communications from 1985 to 1990 during the leadership of Neil Kinnock, becoming one of the first people to whom the term "spin doctor" was applied, and being nicknamed the "Prince of Darkness" because of his "ruthless" and "media-savvy" reputation. Although sidelined during John Smith's leadership from 1992 to 1994, Mandelson became close to Blair and Brown. Mandelson was one of several key people responsible for the rebranding of the Labour Party as New Labour before its victory at the 1997 general election under Blair, to whom he had become a trusted ally and adviser. He was also involved in Labour's campaign for the 2001 general election.

Peter Benjamin Mandelson was born on 21 October 1953 in Hendon, Middlesex, the younger son of George Norman ("Tony") Mandelson (1920–1988), advertising manager of The Jewish Chronicle and a former officer in the Royal Dragoons during the Second World War, and Mary Joyce, daughter of Herbert Morrison, London County Council leader and a cabinet minister in the Attlee government. Of Polish Jewish extraction, his grandfather, Norman Mandelson, founded the Harrow United Synagogue. Mandelson was raised in Hampstead Garden Suburb; recalling his childhood, he said: "my whole upbringing was framed around the Suburb – my friendships and values".

Mandelson attended the Garden Suburb School, and from 1965 to 1972 Hendon County Grammar School. During his teenage years he joined the Young Communist League due to the UK's support of the United States' role in the Vietnam War. He read PPE at St Catherine's College, Oxford. His tutors included Nicholas Stern.

As Chairman of the British Youth Council, in 1978 Mandelson was delegated to attend the Soviet-organised World Festival of Youth and Students at Havana, Cuba. He was elected to Lambeth Council at a by-election in 1979 but stood down in 1982, disillusioned with the state of Labour politics during the leadership of Michael Foot. Mandelson then worked from 1982 to 1985 as a television producer at London Weekend Television on Weekend World, where he formed a friendship with his superior John Birt.

Mandelson is gay and in the late 1990s was said to be "intensely private" about his personal life. Mandelson considers himself a good role model for gay people because of his success in public office. Mandelson married Reinaldo Avila da Silva, a Brazilian translator, on 28 October 2023, after having lived in London with him since March 1998. The couple have a collie dog named Jock, who became popular at parties in the British ambassador's residence during Mandelson's brief tenure in the ambassadorial office.

In 2008 Mandelson was hospitalised, suffering from kidney stone disease, shortly after drinking a glass of Chinese yoghurt in front of reporters in order to show his confidence in Chinese dairy products, which were being banned in Europe due to a concern over deliberate addition with melamine. The melamine was added to boost protein in otherwise diluted milk, but had caused ailments, including deaths, in Chinese children. Though his own condition was unrelated to melamine in dairy products, the media reported on the connection.