The tribunal yesterday decided that his initial sentence was too lenient.
In 2016, Karadzic, 73, was found guilty of war crimes and genocide by a UN tribunal in The Hague and handed a 40-year prison sentence.
He masterminded the Srebrenica massacre of July 1995 – the worst atrocity in Europe since the holocaust in World War Two.
Karadzic had argued his conviction was based on “rumours”.
He launched an appeal last year, telling judges that the ethnic cleansing of Bosniak Muslims and Croats in the 1990s were “myths”.
Karadzic was president of the Bosnian Serb entity Republika Srpska during the Bosnian War in the 1990s.
The tribunal rejected the vast majority of Karadzic’s appeal.
Judge Vagn Joensen said the initial sentence was too lenient, considering the “sheer scale and systematic cruelty” of his crimes.
The war criminal cannot appeal the tribunal’s decision. He sat in the chamber yesterday and did not react to the decision.
The ruling will most likely be one of the final hearings born from the break-up of the former Yugoslavia in the 1990s.
At Srebrenica, Bosnian Serb soldiers massacred more than 8,000 Muslim men and boys in a “safe area” protected by Dutch UN peacekeeping forces.
Judges also said Karadzic was responsible for the siege of Sarajevo, a campaign of sniping and shelling, which lasted more than three years and killed an estimated 10,000 civilians.
After the war, Karadzic went into hiding for years, presenting himself as an “expert” in alternative medicine before his incarceration in Serbia in 2008.
Former Bosnian Serb military army commander Ratko Mladic was also sentence to life in prison for genocide and war crimes in November 2017.