Gbagbo, 65, had been charged with undermining state security.
Her husband, former Ivorian President Laurent Gbagbo, is awaiting trial at the International Criminal Court (ICC).
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 main city, Abidjan.
And it was in the same city where Gbagbo - once called the "Iron Lady" and known for her fiery speeches - faced trial. She was also accused of disturbing public order and organising armed gangs.
She denied witness accounts that accused her of personally distributing arms to the death squads that operated in Abidjan during the five-month conflict.
The court unanimously sentenced her to 20 years, which is the sentence for undermining state security.
The prosecution had only asked for 10 years - but this had only been for the charge of disturbing public order.
The BBC's Ali Bamba in Abidjan says the verdict was delivered in the early hours of Tuesday morning after eight hours of deliberation by the jury.
The former first lady remained calm, as she had been throughout the trial, almost as if she knew what was coming, he says.
Her daughter from a previous marriage, Marie Antoinette Singleton, told the BBC's Newsday programme that 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.
"I was expecting Simone to be acquitted," Rodrigue Dadje told the BBC French service.
"There is no proof of her involvement at a personal levelâ€¦ I am ashamed for the Ivorian justice system."
The former first lady was on trial with 82 other supporters of her husband  - 15 of whom were acquitted.
Two former military allies also received 20-year jail terms: Gen Bruno Dogbo Ble, once head of the Republic Guard, and the ex-navy chief Adm Vagba Faussignaux.
The other accused got lesser sentences.
Before her sentencing, Simone Gbagbo had said that she forgave the prosecution, despite the humiliation she had suffered during the trial.
"I forgive because, if we don't forgive, this country will burn," she said.
Both sides were accused of atrocities in the conflict that followed the 2010 poll.
But our correspondent says despite a promise from President Alassane Ouattara that both sides would face justice, no-one in the president's camp has been charged.
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.

Style is Eternal, which showcases key items from his 50-year career, has been running at the Bowes Museum in County Durham since July.
It has been the most popular exhibition in the museum's history, attracting three time the usual visitor numbers.
The initial closing date of 25 October has now been extended to 8 November.
The show highlights how the designer revolutionised womenswear and includes dresses from the late 1950s which have never been seen in public before.
There have been 55,000 visitors to date, a figure which is expected to reach 70,000 by the time it closes.
Adrian Jenkins, Director of The Bowes Museum said:  "It has been a wonderful moment in the museum's history.
"The exhibition has dazzled visitors and informed us of Yves Saint Laurent's huge contribution to the world of fashion, evident in the way women dress every day."
Ivory Coast's former first lady, Simone Gbagbo, has been sentenced to 20 years in jail for her role in the violence that followed the 2010 elections.
[NEXT_CONCEPT]
The first major UK exhibition of the work of French designer Yves Saint Laurent has proved so popular that it is being extended.