President Serzh Sarkisian signed the agreement to join the Eurasian Economic Union (EES) at a meeting of regional heads of state in the Belarusian capital, Minsk, on October 10.
The Caucasus nation is now set to join Russia, Belarus, and Kazakhstan in the economic bloc when it comes into force on January 1, 2015.
The project builds on the existing Customs Union between Russia, Belarus, and Kazakhstan.
Landlocked Armenia, which is heavily reliant on its ties to Russia, ditched plans last year to sign an agreement on closer trade and diplomatic ties with the European Union.
Also in Minsk, Kyrgyz President Almazbek Atambaev expressed hope that his country would join by year's end.
And President Emomali Rahmon said Tajikistan is analyzing the EES legal documents and does not rule out that it might join.
Earlier, Belarusian President Alyaksandr Lukashenka signed a law on the ratification of the EES, advancing a Russian-led drive to strengthen ties among former Soviet republics.
Lukashenka signed the law hours after both houses of the Belarusian parliament voted to ratify the treaty, which was signed by the presidents of Russia, Kazakhstan and Belarus on May 29.
Lukashenka is hosting an October 10 summit of the Commonwealth of Independent States, which links 11 former Soviet republics.
The summit was attended by 10 of the member states' presidents while Ukraine -- deeply at odds with Russia over Moscow's annexation of Crimea and a conflict with pro-Russian separatists in eastern Ukraine -- was represented by its ambassador to Belarus.
Russia media reports say Lukashenka, Russian President Vladimir Putin, and Kazakh President Nursultan Nazarbaev will meet separately in Minsk.