Lokomotiv Moscow defender Innokenty Samokhvalov died aged 22 following an individual training session.
The Russian giants confirmed the tragic news on Monday after Samokhvalov suffered suspected heart failure.
Samokhvalov – who had been playing for Lokomotiv's reserve team Kazanka in the third division – was training solo amid the coronavirus pandemic.
"We are truly sad to announce the death of Kazanka defender Innokenty Samokhvalov," Lokomotiv said in a statement.
"The footballer became ill during an individual training session. Details of the tragedy are being clarified.
"Samokhvalov is a pupil of Lokomotiv. Kesha came to our Academy in the third grade, won the youth championship of Russia, this season he played for Kazanka. Samokhvalov left a wife and a son.
"Kesha was a kind, helpful person and a good friend. We are truly shocked by what happened and express our sincere condolences to his family and friends."