Mary Elizabeth Truss (born 26 July 1975) is a British politician who was Prime Minister of the United Kingdom and Leader of the Conservative Party from September to October 2022. On her fiftieth day in office, she stepped down amid a government crisis, making her the shortest-serving prime minister in British history. The member of Parliament (MP) for South West Norfolk from 2010 to 2024, Truss held various Cabinet positions under three prime ministers—David Cameron, Theresa May and Boris Johnson—lastly as foreign secretary from 2021 to 2022.

Truss studied philosophy, politics and economics at Merton College, Oxford, and was the president of the Oxford University Liberal Democrats. In 1996 she joined the Conservative Party. She worked at Royal Dutch Shell and Cable & Wireless and was the deputy director of the think tank Reform. After two unsuccessful attempts to be elected to the House of Commons, she became the MP for South West Norfolk at the 2010 general election. As a backbencher she called for reform in several policy areas including the economy, childcare and mathematics in education. Truss co-founded the Free Enterprise Group of Conservative MPs and wrote or co-wrote a number of papers and books, including After the Coalition and Britannia Unchained.

Truss was Parliamentary Under-Secretary of State for Childcare and Education from 2012 to 2014 before Cameron appointed her Secretary of State for Environment, Food and Rural Affairs in a cabinet reshuffle. Although she campaigned for Britain to remain in the European Union, Truss supported Brexit following the outcome of the 2016 referendum. Following Cameron's resignation in 2016 his successor, Theresa May, appointed her Secretary of State for Justice and Lord Chancellor, making Truss the first woman to serve as Lord Chancellor in the office's thousand-year history; in the aftermath of the 2017 general election she was demoted to Chief Secretary to the Treasury. After May announced her resignation in May 2019 Truss supported Johnson's successful bid to become Conservative leader and prime minister. He appointed Truss Secretary of State for International Trade and President of the Board of Trade in July and subsequently to the additional role of Minister for Women and Equalities in September. Johnson promoted Truss to foreign secretary in the 2021 cabinet reshuffle; during her time in the position she led negotiations on the Northern Ireland Protocol and the British response to the Russian invasion of Ukraine.

In September 2022 Truss defeated Rishi Sunak in a leadership election to succeed Johnson, who had resigned because of an earlier government crisis, and was appointed prime minister by Queen Elizabeth II two days before the monarch's death; the government's business was subsequently suspended during a national mourning period of ten days. In response to the rising cost of living and increased energy prices, Truss's ministry announced the Energy Price Guarantee. The government then announced large-scale tax cuts and borrowing, which led to financial instability and were largely reversed. Facing mounting criticism and loss of confidence in her leadership, Truss announced her resignation as leader of the Conservative Party on 20 October. Sunak was elected unopposed as her successor, and appointed prime minister on 25 October. After spending the duration of Sunak's premiership on the backbenches, Truss lost her seat at the 2024 general election.

Mary Elizabeth Truss was born on 26 July 1975 at the John Radcliffe Hospital in Oxford, England. She was the second child of John and Priscilla Truss (née Grasby); the year before Truss's birth, their first son, Matthew, had died. Truss was known by her middle name, Elizabeth, from early childhood, with her father—a professor of pure mathematics at the University of Leeds—using it regularly, which she preferred; after being given a badge with "Mary" on it on her first day of school, Truss asked her teacher that it be changed. She later described her parents' politics as being "to the left of Labour"; her mother, a teacher and nurse, was a member of the Campaign for Nuclear Disarmament. When Truss stood for election as a Conservative, her mother agreed to campaign with her but her father declined to do so. Her parents divorced in 2003.

[S]he stands out in my memory as a sort of strange, unfocused force, hugely in favour of action and change ... it was always hard to see the aim of it all, or where it might lead, except that she would be at the centre of it.

In 1977 Truss and her parents moved to Warsaw in Poland, but returned to Britain after John and Priscilla found it "quite grim". After living briefly in Kidderminster, Worcestershire, the family moved to Paisley in Scotland when Truss was four years old, where she attended West Primary School. In 1985 they moved south to Leeds, where Truss attended Roundhay School; she later said in 2022 that at the school she "saw kids ... being let down", a claim which was criticised as inaccurate by several former Roundhay pupils. When Truss was 12 she and her family spent a year in Burnaby, British Columbia, Canada, where she attended Parkcrest Elementary School whilst her father taught at Simon Fraser University. Truss praised the Canadian curriculum and the attitude that it was "really good to be top of the class", which she contrasted with her education at Roundhay.

Truss's parents had initially wanted her to study at the University of Cambridge, but Truss instead elected to go to Oxford in what her biographers, Cole and Heale, call a "bout of teenage rebellion". She applied to Merton College but was instead pooled to the all-women's St Hilda's College; annoyed, she then complained to both colleges, after which she was accepted by Merton and began her studies there in September 1993. Truss read philosophy, politics and economics and graduated in 1996. During her time at university, Truss was active in the Liberal Democrats and was a member of the Oxford Reform Club. She became the president of the Oxford University Liberal Democrats in her first year and a member of the national executive committee of Liberal Democrat Youth and Students (LDYS) in 1995. During Truss's previous, unsuccessful, bid for the LDYS executive, the party's leader, Paddy Ashdown, said she was "a good debater and is utterly fearless". As a Liberal Democrat, Truss supported the abolition of the monarchy and the legalisation of cannabis, and campaigned against the Criminal Justice and Public Order Act 1994. However, by November 1995 Truss had become critical of the Liberal Democrats, as she "realised the Tory Party was saying quite sane things"; in her last year at the university, she resigned from the LDYS. By 1996 Truss had joined the Conservative Party.