Radovan Karadzic has been convicted of genocide and war crimes during the 1992-1995 war, and sentenced to 40 years in jail.
UN judges in The Hague found the former Bosnian Serb leader guilty of 10 of 11 charges, including genocide over the 1995 Srebrenica massacre.
Radovan Karadzic, 70, is the most senior political figure to face judgement over the violent collapse of Yugoslavia.
His case is being seen as one of the most important war crimes trials since World War Two.
Radovan Karadzic had denied the charges, saying that any atrocities committed were the actions of rogue individuals, not the forces under his command.
Radovan Karadzic’s lawyer said he would appeal, a process that could take several more years.
He faced two counts of genocide.
Radovan Karadzic was found not guilty of the first, relating to killing in several Bosnian municipalities.
However, he was found guilty of the second count relating to Srebrenica, where Bosnian Serb forces massacred more than 7,000 Bosnian Muslim men and boys.
“Karadzic was in agreement with the plan of the killings,” Judge O-Gon Kwon said.
Radovan Karadzic was also found guilty of crimes against humanity relating to the siege and shelling of the city of Sarajevo over several years which left nearly 12,000 people dead.
He can expect to serve two-thirds of his sentence. His time spent in detention – slightly more than seven years – will count towards the total.
Top UN human rights official Zeid Ra’ad al-Hussein welcomed the verdict as “hugely significant”.
Radovan Karadzic said the trial “should give pause to leaders across Europe and elsewhere who seek to exploit nationalist sentiments and scapegoat minorities for broader social ills”.
At least 100,000 people in total died during fighting in the Bosnian war. The conflict lasted nearly four years before a US-brokered peace deal brought it to an end in 1995.
General Ratko Mladic, who commanded Bosnian Serb forces, is also awaiting his verdict at The Hague.