Mr Seselj, leader of the Serbian Radical Party, is charged with 14 counts relating to the Croatian and Bosnian wars of the 1990s.
He has pleaded innocent and filed for acquittal and compensation in March.
While he does not deny making nationalist speeches, he says they did not constitute war crimes.
The BBC's Balkans correspondent Mark Lowen says no defendant has ever been acquitted in the 18-years the tribunal has been running.
Mr Seselj surrendered to the International Criminal Tribunal for the former Yugoslavia (ICTY) in 2003.
He is accused of forming a joint criminal enterprise with the late Serbian leader Slobodan Milosevic to "ethnically cleanse" large parts of Bosnia, Croatia and Serbia's northern Vojvodina region.
The charges against him include murder, torture, sexual assault, forced transportation and destruction of property.
But earlier this year he filed a motion under a specific court rule arguing that the prosecution's evidence was insufficient to support a conviction.
Mr Seselj's Serbian Radical Party has been campaigning for him to be released in time for parliamentary elections next year.
Some fear that if he were released and return to Serbian politics, he could once again galvanise the far right at a time when the country's pro-Western government is trying to move closer to its goal of European Union membership, our correspondent says.
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