The site drew 0.6% more users than in June, when it overtook Mail Online to reclaim its traditional first place, with Telegraph.co.uk taking second place.
Last month Telegraph.co.uk recorded almost 1m fewer visitors than in June, slipping 4.9% to 18.75m.
In terms of UK visitors only, Guardian.co.uk is still top with 8.5m users (41% of its total), ahead of Telegraph.co.uk with 6.4m users (34%) and Mail Online with 5.5m users (29%).
Outside the top three, Times Online was the highest-placed, rising 0.13% from June to 16.4m unique users.
Times Online has 5.9m UK users (36% of its total) while The Sun's group has 5.5m (35%).
At the bottom of the table both the Independent and the Mirror suffered reverses.
Independent.co.uk did not publish how many UK users it has. Mirror Group Digital has 2.96m, which is 57% of its total.
The biggest year on year growth was enjoyed by Telegraph.co.uk, up 108% from its 8.99m figure last July.
The Sun is second in these terms with 69% growth from 9.4m, followed by Mail Online with 58% growth from 11.9m.