Michael Ojo, a Nigerian basketball player who previously played for Red Star Belgrade, died at the age of 27 during a training session in the Serbian capital on Friday, the club said.
"We are deeply saddened by the news", Red Star's spokesperson Igor Vujicin told AFP.
The club couldn't confirm the cause of death, but local media reported that Ojo suffered a heart attack.
Born in Lagos, the 2.16 meter (7ft 1in) center played for Florida State University in the United States before coming to Europe to start his professional career.
After a stint with the Belgrade team FMP Zeleznik, he moved to European giants Red Star, where he spent two seasons, quickly becoming a fan favourite.
Ojo had been without a club since June 30 but was training in Belgrade.
His former team released a statement mourning the loss of the "cheerful giant".
"The sudden and shocking death of Michael Ojo has deeply affected everyone -- players, coaches and the management of a club that accepted him from the first day, and that he considered his own," Red Star said.
EuroLeague also extended condolences to Ojo's family, teammates, coaches, "and many fans who enjoyed his performances".