Slobodan Praljak, 72, tilted back his head and took a swing from a flask or glass as a war-crimes judge at The Hague read out the verdict.
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.
After drinking Praljak told the court: "I am not a war criminal, I oppose this conviction".
Praljak was convicted for his actions between 1992 and 1994, including the rape and murder of Bosnian Muslims in the wake of the dissolution of the former Yugoslavia.
Croatian Prime Minister Andrej Plenkovic later confirmed that Praljak had died, saying in a government tweet, "On behalf of the Government of the Republic of Croatia and on my own behalf, I want to express my deepest condolences to the family of General Slobodan Praljak".
A court guard said early afternoon local time that the defendant was still alive.
Praljak had received a confirmation of his 20-year sentence..
Several convictions for specific crimes for Prlic and the five others were reversed in the appeal, but president Judge Carmel Agius said "all six remain convicted of numerous and very serious crimes".
The Tribunal was set up in 1993, while fighting still raged in the former Yugoslavia, and has indicted 161 suspects for war crimes and convicted 90 individuals.
Last week the tribunal sentenced Gen. Ratko Mladic, the ethnic Serb "Butcher of the Balkans" to life in prison for genocide against Bosnian Muslims.
Wednesday's hearing was the final case at the groundbreaking tribunal before it closes its doors next month.