Kazakh President Nursultan Nazarbaev has signed a law on the ratification of the treaty creating the Eurasian Economic Union (EES), his office said on October 14.
Belarusian President Alyaksandr Lukashenka signed a similar law last week, and Russian President Vladimir Putin did so on October 3.
Under the treaty, the EES will begin functioning as of January 1, 2015.
The project builds on the existing Customs Union linking Russia, Belarus, and Kazakhstan.
Moscow, which is facing Western sanctions over its role in the Ukraine crisis, has been pressuring former Soviet republics to join the EES.
Armenia's president signed an agreement to join the EES during a summit in Minsk on October 10.
Kyrgyzstan's president expressed hope that his country would join by year's end.