Bosnian Serb General Zdravko Tolimir, who was serving a life sentence for genocide, including the massacre at Srebrenica during the early 1990s war in Bosnia, died in The Hague on Monday evening, a court spokesman said.
"He was taken ill at the court's detention centre at around 9pm last night," said spokesman Nenad Golcevski.
"He received medical attention but he passed away."
Tolimir, the 67-year-old former head of military intelligence in the Bosnian Serb Army, was jailed due to crimes including the killing of over 8,000 Muslim men and boys in Srebrenica in 1995, in what remains Europe's worst massacre since World War Two.
He was an important figure of a campaign aiming to carve an ethnically pure Serbian state out of parts of multi-ethnic Bosnia.
Tolimir reported directly to Ratko Mladic, commander of the Bosnian Serb forces, who is still on trial at the International Criminal Tribunal for the former Yugoslavia (ICTY).
There was no announcement on the cause of Tolimir’s death, but he was known for having heart problems, as he was awaiting transfer to a prison in one of the court's sponsoring countries when he died.
The ICTY was set up to try people for war crimes and crimes against humanity committed during the break-up of multi-ethnic Yugoslavia into seven successor states.
The Bosnian War between 1992-1995 left more than 200,000 people dead - mostly Muslim Bosniaks.
Many Serb soldiers, believed to have had an active role in the brutal massacre of 1992, have not yet stood in the face of a judge.