Rosneft Chief Executive Officer Igor Sechin signed a deal to acquire a stake in private Russian company Surgutneftegaz's part of the consortium managing the Venezuelan oil field.
In September, a consortium led by Russian oil companies announced that first oil production started in the block, part of the Orinoco River basin in Venezuela. Gazprom Neft, one of the consortium members, estimated the field holds as much as 50 billion barrels of oil.
State-run news agency RIA Novosti reports that Moscow approved of Rosneft's acquisition, meaning the deal is considered complete.
The project is expected to cost about $25 billion to develop. At its peak, Gazprom Neft estimates it could get about 450,000 barrels of oil per day from the site.
Venezuela has some of the largest deposits of crude oil in the world. The Orinoco belt is considered one of the biggest reserve fields outside of the Middle East.
The health of Venezuelan President Hugo Chavez has raised concerns about the prospects of a struggling oil sector. Chavez is reportedly in Cuba for cancer treatment.