Up to three news stories will be exchanged each day between the Guardian and Salon websites.
Salon is also opening a new Washington bureau to be led by Sidney Blumenthal, former senior aide to President Clinton and a Guardian columnist.
"Salon and the Guardian are both independents that can be fearless in pursuit of the truth," said Patrick Hurley, senior vice president of business operations for Salon.
"Both outlets tend to share a tradition of being at the vanguard of social trends," he told dotJournalism.
"The sophisticated audiences of both Europe and the US are interested in issues globally - and especially between the two countries given current world events."
Salon has around 3 million readers each month and 74,000 paid subscribers. The shared content agreement will allow Salon to reach the Guardian's online audience of nearly 9 million every month.
A Guardian spokesperson told dotJournalism that Guardian Unlimited was popular in the US because of the demand for liberal news - as proven during the Iraq war.
"The quality of writing, quality of journalism and editorial integrity makes Guardian Unlimited an obvious partner for Salon.com," he said.
"Guardian Unlimited has been phenomenally successful - there is no comparison with any other UK newspaper site."
The news offensive kicks off on Wednesday with the online publication of 'The new Pentagon papers' - an article by retired lieutenant colonel Karen Kwiatkowski in which she claims that intelligence was twisted in the rush to war in Iraq.
Salon has also announced a similar content sharing arrangement with Air America, a progressive radio network based in the US.
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