This cuts the Italian oil and gas group's stake in the concession from 90 per cent to 60 per cent.
Rosneft will pay $1.13bn in cash for the concession and will reimburse pro-rata investments already carried out by Eni, which total around $450m.
The Russian oil and gas giant also has an option to buy a further five per cent stake.
Eni discovered the sprawling, "super-giant" Zohr field in the Shorouk concession last year. It contains an estimated 850bn cubic metres of gas.
The approval process for development of the field was completed in February and first gas is expected by the end of 2017.
Eni, whose cash flow fell 19 percent in the third quarter due to low oil prices, has pledged to sell €5bn (£4.2bn) of assets in the next two years.
Rosneft has made headlines in the last week after Glencore and Qatar's sovereign wealth fund agreed to buy a 19.5 per cent interest in the company, which has been dubbed "the largest in the history of Russia".
Glencore and the Qatar Investment Authority will pay €10.2bn to acquire the slice.