Gbagbo, 65, had been charged with undermining state security.
Her husband, former President Laurent Gbagbo, is awaiting trial at the International Criminal Court.
More than 3,000 people died in the violence that followed the presidential poll after the ex-leader refused to accept defeat to Alassane Ouattara.
She and her husband were arrested in 2011 after troops stormed a bunker where the pair had taken refuge in the capital city, Abidjan.
And it was in the same city where Gbagbo – once called the “Iron Lady” – faced trial. She was also accused of disturbing public order and organising armed gangs.
The court unanimously sentenced her to 20 years, twice as long as the prosecutors had asked for.
Her daughter, Marie Antoinette Singleton, told the BBC the sentence was unfair and a sign of “political justice”.
“Why would you want to double it? It’s not about justice, it’s about getting rid of political adversaries.
“If we say that something wrong happened, it happened on both sides. Nobody looked into bringing all responsible parties to trial,” she said.
Simone Gbagbo’s lawyer said they would appeal.
Laurent Gbagbo, is facing four charges at the ICC in The Hague, including murder, rape and persecution.
The ICC had issued an arrest warrant for Simone Gbagbo too, but this was dismissed by the Ivorian government.