The Russian Orthodox Church Abroad has reunited with the Russian Orthodox Church after 80 years of schism sparked by the Bolshevik revolution.
The landmark reunification ceremony was held in Moscow
The move was sealed by Patriarch Alexy II and the head of the Russian Orthodox Church Abroad, Metropolitan Lavry, at an elaborate ceremony in Moscow.
Reunification talks began after the collapse of the Soviet Union in 1991.
But some clergy abroad are rejecting the move. They say that many priests in Russia collaborated with Communists.
Alexy II and Metropolitan Lavry signed the reunification agreement in a ceremony at the Christ the Saviour Cathedral in Moscow.
The Christ the Saviour Cathedral is a symbol of religious rebirth
"By this act, canonical communion within the local Russian Orthodox Church is hereby restored," the act said.
Russian President Vladimir Putin attended the ceremony, shown live on television.
Talks to re-establish ties between the Russian Orthodox Church in exile and its mother church began soon after the collapse of the USSR, which ignited a revival of organised religion in Russia.
A major step was reached when Church leaders in Moscow elevated the murdered Tsar Nicholas II and his family to the status of sainthood in 2000, the BBC's religious affairs correspondent Mike Lanchin reports.
In 2006, the Russian Orthodox Church allowed the remains of the tsar's mother to be reburied in St Petersburg.
One of the last sticking points was disagreement over who would own the extensive property in the hands of the exiled church in the US, Europe and even Israel.
The agreement is that these remain as they are, our correspondent says. The exiles also retain control over the appointment of their own priests.
The Russian Orthodox Church Abroad owes its origin to the civil war - which followed the Bolshevik Revolution of 1917 and ended with the victory of the militantly atheist Communists over the monarchist Whites.
Exiled bishops and clerics proclaimed the Russian Orthodox Church Abroad at a meeting in Serbia in 1922 - later relocating to the United States.
It cut all ties with its mother church in 1927, after the leader of the church in Russia, Patriarch Sergiy, declared loyalty to the Communists.
The New York-based Church says it has nearly 500,000 members.
The Moscow Patriarchate counts nearly 70% of Russia's population of about 142 million as its members.