The Daily Mirror is a British national daily tabloid newspaper. Founded in 1903, it is part of Mirror Group Newspapers (MGN), which is owned by parent company Reach plc. From 1985 to 1987, and from 1997 to 2002, the title on its masthead was simply The Mirror. It had an average daily print circulation of 716,923 in December 2016, dropping to 587,803 the following year. Its Sunday sister paper is the Sunday Mirror. Unlike other major British tabloids such as The Sun and the Daily Mail, the Mirror has no separate Scottish edition; this function is performed by the Daily Record and the Sunday Mail, which incorporate certain stories from the Mirror that are of Scottish significance. The Mirror publishes an Irish edition, the Irish Mirror.

Originally pitched to the middle-class reader, it was converted into a working-class newspaper after 1934, in order to reach a larger audience. It was founded by Alfred Harmsworth, who sold it to his brother Harold Harmsworth (from 1914 Lord Rothermere) in 1913. In 1963 a restructuring of the media interests of the Harmsworth family led to the Mirror becoming a part of International Publishing Corporation. During the mid-1960s, daily sales exceeded 5 million copies, a feat never repeated by it or any other daily (non-Sunday) British newspaper since. The Mirror was owned by Robert Maxwell between 1984 and 1991. The paper went through a protracted period of crisis after his death before merging with the regional newspaper group Trinity in 1999 to form Trinity Mirror (now known as Reach plc). In August 2023 Reach launched a US division of the Daily Mirror, titled The Mirror US.

The Daily Mirror was launched on 2 November 1903 by Alfred Harmsworth (later Lord Northcliffe) as a newspaper for women, run by women. About the name, he said: "I intend it to be really a mirror of feminine life as well on its grave as on its lighter sides ... to be entertaining without being frivolous, and serious without being dull." It cost one penny (equivalent to 57p in 2023).

It was not an immediate success and in 1904 Harmsworth decided to turn it into a pictorial newspaper with a broader focus. Harmsworth appointed Hamilton Fyfe as editor and all of the paper's female journalists were fired. The masthead was changed to The Daily Illustrated Mirror, which ran from 26 January to 27 April 1904 (issues 72 to 150), when it reverted to The Daily Mirror. The first issue of the relaunched paper did not have advertisements on the front page as previously, but instead news text and engraved pictures (of a traitor and an actress), with the promise of photographs inside. Two days later, the price was dropped to one halfpenny and to the masthead was added: "A paper for men and women". This combination was more successful: by issue 92, the guaranteed circulation was 120,000 copies and by issue 269, it had grown to 200,000: by then the name had reverted and the front page was mainly photographs. Circulation grew to 466,000 making it the second-largest morning newspaper.

Alfred Harmsworth sold the newspaper to his brother Harold Harmsworth (from 1914 Lord Rothermere) in 1913. In 1917, the price was increased to one penny. Circulation continued to grow: in 1919, some issues sold more than a million copies a day, making it the largest daily picture paper. In 1924 the newspaper sponsored the 1924 Women's Olympiad held at Stamford Bridge in London.

Lord Rothermere was a friend of Benito Mussolini and Adolf Hitler, and directed the Mirror's editorial stance towards them in the early 1930s. On Monday, 22 January 1934 the Daily Mirror ran the headline "Give the Blackshirts a helping hand" urging readers to join Sir Oswald Mosley's British Union of Fascists, and giving the address to which to send membership applications. By the mid-1930s, the Mirror was struggling – it and the Mail were the main casualties of the early 1930s circulation war that saw the Daily Herald and the Daily Express establish circulations of more than two million, and Rothermere decided to sell his shares in it.

In 1935 Rothermere sold the paper to Harry Guy Bartholomew and Hugh Cudlipp. With Cecil King (Rothermere's nephew) in charge of the paper's finances and Guy Bartholomew as editor, during the late 1930s the Mirror was transformed from a conservative, middle class newspaper into a left-wing paper for the working class. Partly on the advice of the American advertising agency J. Walter Thompson, the Mirror became the first British paper to adopt the appearance of the New York tabloids. The headlines became bigger, the stories shorter and the illustrations more abundant. By 1939, the publication was selling 1.4 million copies a day. In 1937, Hugh McClelland introduced his wild Western comic strip Beelzebub Jones in the Daily Mirror. After taking over as cartoon chief at the Mirror in 1945, he dropped Beelzebub Jones and moved on to a variety of new strips.

During the Second World War the Mirror positioned itself as the paper of the ordinary soldier and civilian, and was critical of the political leadership and the established parties. At one stage, the paper was threatened with closure following the publication of a Philip Zec cartoon (captioned by William Connor), which was misinterpreted by Winston Churchill and Herbert Morrison. In the 1945 UK general election, the paper strongly supported the Labour Party in its eventual landslide victory. In doing so, the paper supported Herbert Morrison, who co-ordinated Labour's campaign, and recruited his former antagonist Philip Zec to reproduce, on the front page, a popular VE Day cartoon on the morning of the election, suggesting that Labour were the only party who could maintain peace in post-war Britain. By the late 1940s, it was selling 4.5 million copies a day, outstripping the Express; for some 30 years afterwards, it dominated the British daily newspaper market, selling more than 5 million copies a day at its peak in the mid-1960s.