A Church spokesperson confirmed the patriarch had died at his residence outside Moscow but gave no further details about his death or funeral arrangements.
The last Soviet president, Mikhail Gorbachev, was among those paying tribute to Alexy II on a state television broadcast.
He was quoted by Interfax news agency as saying, “I am shocked … It is hard to find words. I had immense respect for him.”
After his appointment as leader of the Church in 1990 and against the backdrop of the Soviet Union's demise, Alexy II set about re-establishing the Russian Orthodox as the central moral and spiritual force in society.
Alexy II was born Aleksey Ridiger to a family of German aristocratic immigrants. He entered Leningrad - now St Petersburg - theological seminary in 1947 and graduated in 1949.
He was an outspoken critic of the 1991 coup by the KGB to oust Gorbachev but often had to fight off accusations that he had worked for the KGB.
Around two-thirds of Russia's 142 million people are believed to belong to the Russian Orthodox Church.
