A defense team for former Ivorian President Laurent Ggabgo argued the ICC doesn't have jurisdiction in the case because Ivory Coast isn't a party to the Rome Statute that created the court.
The ICC, however, said there was no "temporal limitation" to a declaration backed by Gbagbo's government accepting limited ICC jurisdiction in 2003.
The court says Gbagbo may bear criminal responsibility for four counts of crimes against humanity for actions during violence that rocked the West African country following contested presidential elections in 2010.
Those elections were meant to unite a country divided by war. Human rights organizations said both parties to the conflict may have committed crimes against humanity.
Gbagbo was captured in Abidjan in 2011 with the help of French peacekeepers and later transferred to The Hague. The ICC last month unsealed a warrant for the arrest of his wife, Simone, on charges she played a role in the post-election violence.