Norwegian shipping tycoon John Fredriksen announced the deal, which will see Rosneft book "a significant portion" of the company's idle rigs, at the St. Petersburg Economic Forum on Saturday.
“We have sought to access the growth opportunity represented by the Russian market for several years,” NADL chief executive Alf Ragnar Lovdal, said in a statement. "After the closing of this transaction, will have created a powerful force in the Russian market and for the Arctic region."
He stressed that Seadrill, which currently has a 70 percent stake, would, at least after the "initial transaction", remain the largest shareholder in NADL.
"The agreement contemplates Rosneft acquiring a significant stake in North Atlantic Drilling Ltd. with the latter entering Russia's onshore drilling market, and the signing of long-term contracts for onshore and offshore drilling," Rosneft said in a statement on Saturday.
Rosneft chairman Igor Sechin told the Rossiya 24 TV channel that his company could take a stake of up to 50 percent in Seadrill over time.
"We're becoming Seadrill shareholders. At this stage we're discussing combining our service company RN Bureniye with Seadrill and handing it some of our orders. In time we could increase our stake to 50 percent," he was quoted as saying by the Interfax news agency.
However, Norway's Dagens Næringsliv business newspaper on Monday reported that Sechin had been referring to NADL rather than its parent company Seadrill.
Norway's richest man, Jon Fredriksen, is Seadrill's largest shareholder with a stake of 24.54 percent.
Seadrill spun NADL off in January, listing a 30 percent stake on the New York Stock Exchange.
According to Interfax, the deal values North Atlantic Drilling at around $2 billion.