"We have gained access to two vast and extremely promising areas in Russia, a great hydrocarbon producer," Eni Chief Executive Officer Paolo Scaroni said in a statement.
Both sides will set up joint ventures for offshore production. Eni will have a 33.3 percent stake in each.
The Italian energy company estimates there are at least 36 billion barrels of oil equivalent in recoverable reserves in the deposits on Russia's continental shelf.
Meanwhile, Rosneft said it was entering into talks with Russian independent oil producer Lukoil to start work on the arctic shelf.
"We have confirmed out interest but these are just initial contacts," said the head of Lukoil, Vagit Alekperov, in comments quoted by Russia's state-run news agency RIA Novosti.
Financial terms of the deals weren't disclosed to the public.