Gbagbo’s trial started in January 2016, but three years later judges said they had found that prosecutors had “failed to satisfy the burden of proof to the requisite standard.”
It was one of the ICC’s biggest failures since it was set up in 2002 as the world’s only permanent war crimes court.
Prosecutors then appealed, focusing on what they said was a procedural error when judges in 2019 initially handed down the decision orally, and did not issue a full written judgement until months later.
But the ICC dismissed the prosecution arguments on Wednesday, saying it was “self-evident” the verdict had been the same whatever the timing of the written judgement had been.
The appeals judges also upheld the original findings that the evidence offered against Gbagbo was “exceptionally weak”.
The verdict on Wednesday was being closely watched in Ivory Coast, where Gbagbo’s shadow still hangs over a nation that remains mired in political crisis.
Gbagbo was president from 2000 to 2010, a time of turmoil in the world’s top cocoa grower, formerly a haven of peace and prosperity in troubled West Africa.
His old foe Ouattara ignited fresh unrest last year when he announced he would seek a third term in office — a scheme that critics said sidestepped constitutional limits.
Clashes claimed 87 lives while most of the opposition snubbed the October ballot — which Ouattara won by a landslide.
But after Ouattara offered to give Gbagbo passports to aid his return, Gbagbo’s Ivorian Popular Front (FPI) party ended a years-long electoral boycott.
Wednesday’s judgment was key for the credibility of the ICC after a string of high-profile failures and controversies over the fact that it has largely focused on African suspects.
Outgoing ICC prosecutor Fatou Bensouda suffered a series of setbacks including the Gbagbo case and the acquittal on appeal of DR Congo former vice president Jean-Pierre Bemba.
Kenyan President Uhuru Kenyatta also saw charges of crimes against humanity over electoral bloodshed dropped by Bensouda.
Bensouda meanwhile is under US sanctions for an ICC probe into Afghanistan, while her investigation into the Palestinian territories has enraged Israel and Washington.