Croatian state TV says Slobodan Praljak has died after taking poison at war crimes court in The Hague.
On hearing that his 20-year jail term had been upheld, the ex-commander of Bosnian Croat forces said he "was not a criminal" and then drank from a bottle.
"I am not a war criminal", Slobodan Praljak said before throwing back his head with the liquid, eliciting a gasp from a judge at the United Nations court in The Hague, Netherlands.
Tribunal spokesman Nenad Golcevski, when asked by AP if he could confirm the death, said: "I have no information to share at this point".
The Guardian reports Praljak, 72, has died in a local hospital where he was rushed from the courtroom in the southern Bosnian town of Mostar, south of Sarajevo.
Presiding judge Carmel Agius immediately suspended the proceedings and an ambulance was called.
Jordan King warns USA moving embassy to Jerusalem could jeopardize peace efforts
In just a few more weeks, the president will decide whether to approve the move or stave off the decision for another six months. On June 1, Trump shied away from what was a major campaign promise by signing a legal waiver that keeps the embassy in Tel Aviv.
He and six other Bosnian Croat political and military leaders had appealed against their 2013 convictions for involvement in the crimes.
Praljak was a former assistant minister of defense of Croatia, and later commander of the main staff of the former Croatian Defense Council, a militia operating in Bosnia.
The officials were convicted four years ago of participating in an "ethnic cleansing" campaign against Bosnian Muslims during Bosnia's war.
As Slobodan Praljak listened to his sentence of 20 years, he raised a small bottle to his lips in full view of the camera crew and shouted, "I am not a war criminal, I reject the verdict with contempt!"
Wednesday's hearing was the final case at the groundbreaking tribunal before it closes its doors next month.