Zdravko Tolimir, 67, died last night at the court's detention unit in The Hague, said a representative of the tribunal's press office who declined to give her name because the court had not yet made an official announcement.
Tolimir, the Bosnian Serb Army's top intelligence officer, was convicted in December 2012 of genocide and other crimes in the massacre by Bosnian Serb forces of some 8,000 Muslim men in Srebrenica, eastern Bosnia Europe's worst mass killing since World War II. He was sentenced to life imprisonment.
The judge said Tolimir had "full knowledge of the despicable criminal operations" of Bosnian Serb forces that carried out the massacre.
Appeals judges upheld most of his convictions last April and confirmed his sentence.
Despite the genocide conviction, Tolimir was considered a hero among Bosnian Serbs.
In a statement, Bosnian Serb Prime Minister Zeljka Cvijanovic called him, "an exceptional man, brave and honorable general who ... Contributed greatly to the defense of his people" and the Serb-controlled part of Bosnia.
Mladic's trial is still underway, while verdicts in Karadzic's case are expected to be delivered early this year.
Both men are charged with genocide and other crimes allegedly committed during the 1992-95 Bosnian war that left 100,000 people dead.
