Belarus officially joined the Shanghai Cooperation Organization (SCO) on Thursday, which was founded by Russia and China to deal with international security issues.
Russian President Vladimir Putin, his Chinese counterpart Xi Jinping and other state leaders signed the documents admitting Belarus at the SCO summit in the Kazakh capital Astana.
Kazakh President Kassym-Jomart Tokayev, who is chairing the SCO this year, congratulated Belarusian ruler Alexander Lukashenko, who is regarded as Europe's last dictator.
Putin held talks with several heads of state, including Xi, after his arrival in Astana on Wednesday.
According to Chinese state television, Xi once again assured Putin at their bilateral meeting that he would continue to work towards a peaceful solution to the war in Ukraine.
Xi reaffirmed China's cooperation with Russia, which invaded neighbouring Ukraine over two years ago.
Kremlin spokesman Dmitry Peskov described the talks with China as very successful.
Both sides had agreed that negotiations in the conflict over Ukraine made no sense without Russia, Peskov said on Russian state television.
Russia was not invited to the peace summit initiated by Ukraine in Switzerland in mid-June. China, which has presented its own peace plan to end the war, also stayed away from the meeting.
According to the Kremlin, Putin also discussed the Ukraine war in Astana with his Turkish counterpart Recep Tayyip Erdoğan, who has repeatedly acted as a mediator.
Erdoğan reiterated an invitation for Putin to visit Turkey. Turkey is currently not a member of the SCO.
In the course of his war against Ukraine and ensuing tensions with the West, Putin has repeatedly declared his ambitions for a new world order - in which the United States does not play a dominant role.
India, Pakistan, Iran and the Central Asian states of Kyrgyzstan, Tajikistan and Uzbekistan are also members of the SCO, which was initially founded in 2001 to combat terrorism.
Putin is unlikely to hear much criticism of his invasion of Ukraine from the pro-Russian group.
Indian Prime Minister Narendra Modi is not attending the summit; according to the Kremlin, he plans to visit Russia afterwards.
