The trial began five months ago but had been adjourned for a second time one month ago pending the appeal. It will resume on April 13 when prosecutors start calling witnesses.
During his opening statement last month, Karadzic denied involvement in the two worst atrocities of the 1992-95 Bosnian war - the four-year siege of Sarajevo by Serb forces, where 10,000 died, and the killing of more than 7,000 Bosnian Muslim men and boys at Srebrenica in 1995.
In all, Karadzic faces 11 charges of genocide, crimes against humanity and violating the laws and customs of war. He denies them all.
The appeals judges said in their ruling that his demand for more time was groundless. Karadzic had boycotted the start of the trial last year, arguing that he needed more time to prepare.
Mindful of the way that the late Serbian president Slobodan Milosevic was able to drag out his war crimes trial at the Hague, the court has appointed London barrister Richard Harvey as Karadzic's legal adviser to step in if he boycotts or obstructs proceedings.
The tribunal said it expects to hold trial hearings three days a week once it restarts.
Karadzic stepped down as leader of Bosnia's Serbs in 1996 and went into hiding until he was captured in Belgrade in 2008, bearded and disguised as an alternative healer.
