Last month the Guardian unveiled a beta version of the responsive site, which at the time was described as a "work-in-progress" and "still in its early stages".
Now the site is the default for mobile users and , although they can still choose to view the desktop version instead if they wish.
In a post published on the Guardian today, Guardian News & Media said the responsive site is "designed to optimise visually across all smartphones and small tablet devices".
It added that the launch "follows a successful one-month beta trial of the new responsive site via beta.guardian.co.uk".
Product manager Chris Mulholland told Journalism.co.uk that the news outlet assessed use of the beta site by an audience sample of 700,000 people, and received written feedback from 450 people.
He said that feedback had been "really positive", and that people "liked the way the site worked".
The site launched today is "the first step and we hope it will evolve into the perfect product", he added.
As well as resizing to fit the user's screen size, the site also responds to connection speeds, the Guardian post explains.
"As the use of mobiles and tablets continues to grow rapidly, so does the range of devices and screen sizes on which people are reading us", chief digital officer Tanya Cordrey added.
"Our own mobile traffic has grown an incredible 63 per cent year-on-year and, in line with our digital first strategy, we want to ensure our readers have the best possible experience when looking at the Guardian website on any type of small tablet or mobile."
According to the Guardian, traffic from smartphones and tablets now makes up more than 30 per cent of the total.
Speaking to Journalism.co.uk earlier this month for a podcast on the subject of responsive design, front-end architect at the Guardian Andy Hume identified that "the diversity of the web as a platform is only growing".
"There's real divergence of devices at the moment ... that long-tail of devices is growing very very fast and I think the only way to deal with that practically is with a responsive design approach, rather than trying to target these individual platforms".