Alexei Leonov, a legendary Soviet cosmonaut who was the first man to perform a spacewalk in 1965, died in Moscow on Friday aged 85 after a long illness.
The Russian space agency Roscosmos said it was saddened to announce the death of "cosmonaut No 11" who was twice decorated with the country's top honour, the Hero of the Soviet Union.
Tributes poured in from across the globe, with Western space agencies and experts praising Leonov's contribution and admirers bidding him farewell.
"This is a loss for the whole planet," said Russian cosmonaut Oleg Kononenko.
Leonov's assistant Natalia Filimonova told AFP that he died at Moscow's Burdenko hospital after a long illness.
President Vladimir Putin sent his condolences to Leonov's loved ones, Kremlin spokesman Dmitry Peskov said.
"They knew each other very well," Peskov said. "Putin always admired Leonov's courage."
British astronaut Tim Peake described the Russian as "a hero to so many" who was "always full of wisdom and humour."
"I filmed the Earth, perfectly round, the Caucasus, Crimea, the Volga. It was beautiful."