AP
BuzzFeed's Ben Smith was the first to report the news.
"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.
"Because this news leaked before we were prepared to announce it, I'm not yet able to provide any details of this momentous new venture, but it will be unveiled very shortly."
Greenwald has been one of the main players in the
In late August, his partner, David Miranda, was detained for nine hours at London's Heathrow Airport earlier this month under a U.K. anti-terror law. Authorities said he was carrying some 58,000 sensitive government documents upon being detained.
Jennifer Lindenauer, a Guardian spokeswoman, said the paper was sorry to see Greenwald leave the publication.
"Glenn Greenwald is a remarkable journalist and it has been fantastic working with him," she said in a statement.
"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. We are of course disappointed by Glenn's decision to move on, but can appreciate the attraction of the new role he has been offered. We wish him all the best."