People now have to pay to read The Sun newspaper online.
The online version is called Sun+ and costs subscribers £2 per week. The daily print tabloid costs £3.20 a week.
Rupert Murdoch's News UK, which owns The Sun, announced the paywall in March, saying free online access had become "untenable".
The Sun has a circulation of 2.3 million in print, while about 1.7 million people visit its website each day.
The Times, also owned by News UK, introduced its paywall three years ago. It requires a paid subscription before any stories can be viewed.
In March, the Daily Telegraph introduced a so-called metered paywall.
Sun+ will try to attract subscribers with offers such as video of all Premier League goals.