Rebekah Brooks, the chief executive of The Sun’s owner, News UK, said the site will go free from 30 November, which is timed to fall on Cyber Monday, one of the biggest online shopping days of the year.
Her decision is an admission that the paper’s paid-for strategy, which began in 2013, has not worked as The Sun fell badly behind free rivals such as MailOnline, The Guardian, the Mirror titles and BuzzFeed which have won huge global audiences in the tens of millions each.
Sources said the move is not just about increasing online advertising but also offering advertisers a "cross-media" audience across both print and online.
The Sun has 225,000 paying subscribers while MailOnline claims 218 million monthly unique browsers.
Current subscribers to The Sun will not be charged during November and those with quarterly or annual subscriptions will receive a refund.
Brooks said The Sun will be "predominantly free" as its "classic PDF" app, an electronic replica of the newspaper, will still be paid-for and the premium features of its fantasy football game, Dream Team, will also require payment.
The Sun is dropping separate content for smartphone and tablet apps in favour of a single product that works across online and mobile.
Brooks said in a staff memo: "Entering this new chapter for The Sun, we are in a strong position thanks to the many learnings we bring from the paid-for era. We know more about our readers than ever before."
She added that the acquisition of viral video company Unruly and social news content firm Storyful "will play a big role in how we supercharge our digital advertising capabilities" at The Sun.
"We believe taking this step will further our prospects for long-term growth, drive larger audiences for our valuable content in the UK and Ireland, and help preserve our ability to create great journalism for our readers for years to come," said Brooks.