features US and world news, along with the familiar technology, science, books, travel and sports sections of Guardian Unlimited.
A number of user services tailored to the US market will be introduced over time, including holidays and dating, as well as recruitment advertising.
The site launched with an exclusive interview with Hillary Clinton by Michael Tomasky, in which she criticised George Bush for applying the same "broad brush" approach to a range of different terrorist groups.
Alan Rusbridger, editor of the Guardian, said: "There is clearly a demand in the US for the Guardian perspective on global news and comment.
"Under the editorship of the talented political commentator and journalist Michael Tomasky, Guardian America will enable us to meet that need, providing a discerning US audience with quality multimedia journalism, and the very best comment and analysis."
Tomasky was appointed editor of Guardian America in May this year. He said: "The Guardian's already-huge US audience shows that they must be doing something right.
"What we'll try to do is to bring those American readers the best and most relevant of what London produces every day and combine it with political and cultural coverage of the states that is more geared toward a US audience's interests but staying faithful to the Guardian sensibility."
Guardian America's editorial team of eight is based in Washington.
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