Gibson, who will lead a team based in New York, will also be joined by guardian.co.uk news editor, Stuart Millar, who will be appointed deputy editor of the new operation.
The Guardian has an audience of around 8 million unique users in the US (source: Comscore Feb 2011) and is targeting further growth.
Commenting on her appointment, Gibson said: "We're immensely proud of guardian.co.uk and how its global audience has grown, especially with our coverage of WikiLeaks and the Middle East over the last few months. We've learned a lot about what digital journalism can do, and now we want to build on that to cover America better and cover the world better for our American users. To do that, we'll be mixing the journalistic ambition, trust and long history of the Guardian with the energy and innovation that comes from being a start-up in the world capital of digital media."
Alan Rusbridger, editor-in-chief, Guardian News & Media, said: "The Guardian, which is owned by the Scott Trust, has attracted a very considerable audience in America in recent years. We believe there is real demand for the sort of open, internationalist, digital journalism which we have been pioneering. Janine Gibson has been at the heart of our web operations and is the ideal person to lead our new team in the US."
Gibson is the second high-profile US appointment to be announced by the Guardian in recent weeks. Steve Howe, formerly president of the FT in the US, joined last week as chief revenue officer.
The appointments have been announced as the Guardian prepares to stage its first US Guardian Activate conference at the Paley Center for Media in New York on April 28th. The technology summit is designed to bring together the world's brightest and most influential figures to debate how technology is driving positive social change.
Janine Gibson began her journalistic career working for the monthly TV trade magazine, Televisual, first as staff writer, then as news editor and finally deputy editor. She joined the weekly trade newspaper Broadcast in July 1997 as international editor and moved to the Independent as media correspondent a year later.
In 1998 she joined the Guardian as media correspondent. In March 2000, she was appointed editor of MediaGuardian and launch-edited the section's spin off website mediaguardian.co.uk. In 2003 she became responsible for the Guardian's weekly G3 specialist sections.
In Feb 2007 she moved back to the website as executive editor, becoming editor of guardian.co.uk in November 2008. The site now reaches 40 million monthly unique users, with an average of 2.4m a day, says the Guardian.
