Lokomotiv Moscow defender Innokentiy Samokhvalov died during an individual training session forced by the coronavirus lockdown, the Russian club announced on Monday.
"He felt bad during an individual training session, the circumstances (surrounding his death) are still being clarified," Lokomotiv said in a statement.
"He was a kind, nice man, a good friend. It's deeply sad for our family."
Samokhvalov, 22, arrived at Lokomotiv in 2015 but is yet to make an appearance in the Russian Premier League. He currently plays for Lokomotiv's reserve team Kazanka in Russia's third tier.
His coach at Kazanka, Alexander Grishin, said that Samokhvalov had suffered a "cardiac arrest".