Greenwald, an American who lives in Brazil, declined to offer details of his plans, saying they were leaked prematurely.
"My partnership with the Guardian has been extremely fruitful and fulfilling: I have high regard for the editors and journalists with whom I worked and am incredibly proud of what we achieved," Greenwald said in a statement.
"The decision to leave was not an easy one, but I was presented with a once-in-a-career dream journalistic opportunity that no journalist could possibly decline."
But Greenwald told BuzzFeed it would be "a very well-funded... Very substantial new media outlet."
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"My role, aside from reporting and writing for it, is to create the entire journalism unit from the ground up by recruiting the journalists and editors who share the same journalistic ethos and shaping the whole thing -- but especially the political journalism part -- in the image of the journalism I respect most," he was quoted as saying.
BuzzFeed said Greenwald will continue to live in Rio de Janeiro and would bring some staff to Brazil but the new organization's main hubs will be New York, Washington and San Francisco.
Greenwald, a lawyer by training, worked with former National Security Agency contractor Edward Snowden on a series of reports based on documents revealing a vast electronic surveillance program by the United States, in cooperation with Britain.
The Guardian's Jennifer Lindauer said in a statement accompanying Greenwald's that he "is a remarkable journalist and it has been fantastic working with him."
"Our work together over the last year has demonstrated the crucial role that responsible investigative journalism can play in holding those in power to account," she said.