A former professional boxer, Anders Hugger Nielsen, 34, was found dead on Sunday afternoon at the Olympia boxing club in Odense where he worked as a coach.
The police investigating the case have not revealed any further details.
After a successful career as an amateur, Hugger boxed professionally for Stable Palle in 2006.
He only boxed 11 fights, all of which he won, before an eye injury put an end to his promising career in 2009.
“It is terrible. I was almost nauseous when I found out,” Rudy Markussen, a Danish boxer who used to train with Hugger, told Ekstra Bladet.