The Sun is a tabloid newspaper published in the United Kingdom and the Republic of Ireland. As a broadsheet, it was founded in 1964 as a successor to the Daily Herald; it became a tabloid in 1969 after it was purchased by its current owner. It is published by the News Group Newspapers division of News UK, itself a wholly owned subsidiary of Rupert Murdoch's News Corp. Since The Sun on Sunday was launched in February 2012, the paper has been a seven-day operation. The Sun previously had the largest circulation of any daily newspaper in the United Kingdom, but it was overtaken by rival Metro in March 2018.

In 2012, The Sun on Sunday was launched to replace the closed News of the World, employing some of its former journalists. The average circulation for The Sun on Sunday in January 2019 was 1,178,687.

In January 2019, it had an average daily circulation of 1.4 million. The Sun has been involved in many controversies in its history, including its coverage of the 1989 Hillsborough football stadium disaster. Regional editions of the newspaper for Scotland, Northern Ireland and the Republic of Ireland are published in Glasgow (The Scottish Sun), Belfast (The Sun) and Dublin (The Irish Sun) respectively. There is currently no separate Welsh edition of The Sun; readers in Wales get the same edition as readers in England.

On 12 and 13 June 2014, to tie in with the beginning of the 2014 World Cup football tournament, a free special issue of The Sun was distributed by the Royal Mail to 22 million homes in England.

The boycott in Merseyside following the newspaper's coverage of the Hillsborough disaster in 1989 meant that copies were not dispatched to areas with a Liverpool postcode. Royal Mail employees in Merseyside and surrounding areas were given special dispensation by their managers to allow them not to handle the publication "on a case by case basis".

The main party leaders, David Cameron, Nick Clegg and Ed Miliband, were all depicted holding a copy of the special issue in publicity material.[151] Miliband's decision to pose with a copy of The Sun received a strong response. Organisations representing the relatives of Hillsborough victims described Miliband's action as an "absolute disgrace" and he faced criticism too from Liverpool Labour MPs and the city's Labour Mayor, Joe Anderson. A statement was issued on 13 June explaining that Miliband "was promoting England's bid to win the World Cup", although "he understands the anger that is felt towards the Sun over Hillsborough by many people in Merseyside and he is sorry to those who feel offended."

Promoted as "an unapologetic celebration of England", the special issue of The Sun ran to 24 pages.

The Sun has dominated the circulation figures for daily newspapers in the United Kingdom since the late 1970s, at times easily outpacing its nearest rivals, the Daily Mirror and the Daily Mail. For a brief period in the late 1990s and early 2000s, this lead was more than a million copies per day. Sustained decline began in 2004, in line with print journalism as a whole, and it lost more than a million copies from its daily figures in the six-year period from 2012–18. The Sun's long run at the top was finally broken in February 2018 when it was announced that the circulation of the free Metro newspaper had overtaken it for the first time. However it remains the biggest-selling newspaper in the UK.