Rosneft chairman Igor Sechin and Pietro Barbaro Group owner Giovanni Barbaro announced the agreement on Friday.
It will see the Russian giant buy 95% of the shares of Prime Shipping, with Barbaro retaining 5%.
Prime carries oil and products in the Black Sea and the Caspian Sea, plus the Volga and Don river systems.
Rosneft is teaming up with Sberbank Merchant Banking on the takeover.
