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.
Slobodan Praljak, 72, was one of six former Bosnian Croat political and military leaders up before the court.
He committed suicide when Carmel Agius, the presiding judge, read out a verdict rejecting his appeal.
Slobodan Praljak brings a bottle to his lips, during a Yugoslav War Crimes Tribunal in The Hague, Netherlands..
Ignoring a request to sit down, he swigged from what appeared to be a small flask and announced: "What I drank was poison".
The 72-year-old former commander of the Bosnian forces during the 1992 to 1995 conflict was one of six military and political officials receiving appeal sentences Wednesday, which was due to be the last session of the global tribunal set up by the United Nations in 1993 with the motto "bringing war criminals to justice and justice to the victims".
In their ruling, the judges allowed part of Praljak's appeal, saying the bridge had been a legitimate military target during the conflict.
All had been convicted in 2013 of persecuting, expelling and murdering Muslims during Bosnia's war.
More than an hour after the incident, a court guard told Reuters news agency said Praljak was still "being treated".
A total of 100,000 people died and 2.2 million were displaced in the three-year war. When I first met him it was on the front line in Croatia in 1991, and as soon as the camera crew arrived all hell erupted.
Croatian PM Andrej Plenkovic said: "We have all unfortunately witnessed his act by which he took his own life".
According to Croatian media, reporting on Wednesday afternoon, Praljak - who during the war in Bosnia-Herzegovina in the 1990s headed the Main Staff of the Croat Council of Defense (HVO) - died in the hospital.
Praljak was also said to have inflicted cruel treatment on Bosnian Muslims, by arranging for their expulsion and forced transfer and by submitting those imprisoned to forced labour. They also had overturned some of his convictions, but refused to reduce his overall sentence.
A 20-year term was upheld against former military leader Milivoj Petkovic, 68, while a 16-year sentence was confirmed against ex-military police chief Valentin Coric, 61, and and 10 years for former police official Berislav Pusic, 65.
The appeals judges upheld a key finding that late Croat President Franjo Tudjman was a member of a plan to create a Croat mini-state in Bosnia. The six surrendered with Croatia under pressure to comply with the court in return for joining the European Union.