The brother-in-law of the late Rwandan President Juvenal Habyarimana, also known as "Z", had been found guilty of genocide and extermination by the lower court in 2008 and sentenced to 22 years in jail.
The five appeals court judges were unanimous to conclude that there had been "serious errors" in his trial and that his conviction violated "the most basic principles of justice".
After the reading of the ruling, Zigiranyirazo's counsel John Philpot restated his point of view claiming that all charges brought against his client had been "made-up" after his arrest in Belgium on June 26, 2001.
"It's a lesson for International Justice. They should gather evidence before arresting people", the Canadian lawyer said during a Press conference on Tuesday.
Protais Zigiranyirazo, who is in his seventies, expressed his joy to be released and paid homage to the "Victory of truth against lies". He added: "I have been arrested and imprisoned for nothing; I lost my belongings; my family is scattered; my children have no future".
Zigiranyirazo told the BBC he would be seeking compensation for his eight-and-half years in detention.
Tuesday morning, as he was visiting the tribunal, he happened to meet in a corridor the Chief Prosecutor of the ICTR, Hassan Bubacar Jallow, who wished him "good luck" and congratulated him for his new condition as a "free man".
In Kigali, the daily New Times quoted Rwandan minister of Justice Tharcisse Karugarama saying : "Rwanda has legally committed itself to accept decisions by all courts whether national, regional or international. However this does not mean that we welcome the decision to acquit ‘Z.' It is a sad day for Rwanda, it is a sad day for genocide survivors to see an organizer of the genocide acquitted".
It's the first time in the ICTR history that an accused convicted by the lower court is acquitted on appeal.
© Hirondelle News Agency