Praljak was one of six former Bosnian Croat political and military leaders who found themselves in front of the worldwide war crime tribunal.
As judges at the International Criminal Tribunal for the Former Yugoslavia were delivering rulings on Wednesday on appeals related to Croatia's involvement in the 1992-95 Bosnian conflict, one of the six defendants, Praljak, who was standing, addressed the court.
Croatian state TV said Slobodan Praljak, 72, a former commander in Bosnia's 1992-95 war, appeared to drink from a small bottle moments after judges at the global criminal tribunal in The Hague reconfirmed a 20-year sentence.
A Bosnian-Croat war chief has died after downing poison during his war crimes trial at The Hague, the Croatian Prime Minister has confirmed.
And a Serbian lawyer who has frequently defended suspects at the United Nations war crimes court in the Netherlands said it would be easy to slip poison into the courtroom. "I am not a war criminal".
After Praljak drank the poison, presiding judge Carmel Agius immediately suspended the proceedings and an ambulance was called. Praljak then proceeded to drink from the glass and announced that he had taken poison, according to the BBC.
Slobodan Praljak was one of six former political leaders up before the court. Source AAP
Praljak was sentenced to 20 years in jail along with his co-conspirators back in 2013, though it is not clear if he began serving that sentence before his appeal.
Although they were allies against the Bosnian Serbs in the Bosnian War of 1992 - 1995, Bosnian Croats and Muslims were also in conflict for nearly a year, with Mostar seeing some of the bloodiest conflict as the country of Yugoslavia messily divided.
The crimes Praljak were convicted for stem from the destruction of a 16th century bridge in November of 1993, which the judges said "caused disproportionate damage to the Muslim civilian population".
Wednesday's hearing is the final case to be completed at the groundbreaking tribunal before it closes its doors next month.
Two other suspects had also had their sentences upheld before the hearing was halted. It's also important to note that while the courts overturned some of Praljak's convictions, they refused to reduce his sentence.