South Stream is expected to deliver 30bn cubic metres of gas a year from Russia
Slovenia has signed a deal with Russia for a massive gas pipeline from Russia to Europe to run through its territory.
The South Stream accord was signed by the Slovenian and Russian cabinet ministers in Moscow.
Russian Prime Minister Vladimir Putin said Moscow now had deals with all the key European partners in the project.
South Stream - which is scheduled to be completed by 2015 - is in competition with a European Union-backed pipeline - Nabucco.
Bulgaria, Serbia, Hungary and Greece have alrady agreed to take part in South Stream, which will deliver gas from Central Asia and Russia to Central Europe and Italy through the Balkans.
The EU imports a quarter of its natural gas from Russia, mostly via Ukraine.
But disputes between Kiev and Moscow have disrupted supplies to Europe in recent years.