"Legendary cosmonaut" Alexei Arkhipovich Leonov, the first person to walk in space, has died in Moscow aged 85.
Russian space agency Roscosmos confirmed his death on its website, but gave no further details.
Mr Leonov performed his spacewalk on 18 March 1965, when he exited his Voskhod 2 capsule, secured by a tether.
Roscosmos said in its statement: "The legendary Soviet cosmonaut No 11 was the first human in the world to perform a spacewalk, was twice awarded with Hero of the Soviet Union title."
It said he was "one of the first cosmonauts of the world space era" who was "committed to his Motherland and his cause".
"His name is lettered in gold in the world space exploration history," the space agency said.
On his second trip to space 10 years later, Me Leonov commanded the Soviet half of the Apollo-Soyuz 19 mission.
It was the first joint space mission between the Soviet Union and the United States, carried out at the height of the Cold War.
NASA also paid tribute to the legendary cosmonaut.
The US space agency tweeted: "We're saddened by the loss of legendary @roscosmos cosmonaut Alexei Leonov who became the first human to walk in space on March 18, 1965.
"His venture into the vacuum of space began the history of extravehicular activity that makes today's @Space_Station maintenance possible."
Mr Leonov turned 85 in May. Days before his birthday, two Russian crew members on the International Space Station ventured into open space on a planned spacewalk with stickers attached to their spacesuits paying tribute to the cosmonaut, and congratulated him from space.
Roscosmos said Mr Leonov's funeral will take place on Tuesday at a military cemetery outside the Russian capital.