The formerly rival churches will seal their historic reconciliation at a ceremony on May 17 in the Russian capital's largest cathedral.
President Vladimir Putin has hailed the move, which will end more than eight decades of bitter estrangement, as an "epoch-making event."
The head of the Russian Orthodox Church, Patriarch Aleksy II, and Metropolitan Lavr, the leader of the New York-based Church Abroad, are due to sign an "act of restoration of canonical relations."
Clergymen from both churches will then celebrate a lavish joint Mass in Moscow's Christ the Savior Cathedral. Moscow's largest cathedral was blown up by Soviet dictator Josef Stalin and rebuilt after the 1991 Soviet collapse.
End To A 'Difficult History'
Metropolitan Kirill of Smolensk and Kaliningrad, the head of external relations for the Moscow Patriarchate, expressed hope that the event will heal a painful rift.
"This is a historic event that we hope, by God's grace and the prayers of Russia's New Martyrs [repressed under Soviet rule], will put an end to the long and difficult history of the division of our people," he said.
Metropolitan Lavr (pictured right) was equally upbeat as he arrived in Moscow on May 15 with dozens of priests and choristers.
"I think it's a very important event, of course, and it is particularly important for the Russian Church, which has been divided for 90 years," he said.
The reunification is an important step for Russia in coming to terms with its communist past.
Founded by clerics who fled the 1917 Bolshevik Revolution, the breakaway Orthodox church was first based in Stavropol, a southern Russian city then controlled by the White Army.
As the Red Army advanced, the church moved to Ottoman Turkey, and then to Serbia, before establishing its headquarters in New York.
Church 'Stained' By Soviet Past
The Church Abroad severed all ties with Moscow in 1927 after the Moscow Patriarchate signed a declaration of loyalty to the atheist Soviet regime.
Contacts were only officially renewed in 2003, when the Moscow Patriarchate recanted the declaration as a "tragic compromise."