0338 GMT September 28, 2020
The Senegalese, 87, faced corruption and money-laundering charges linked to the Russian doping scandal, BBC Sport reported.
Diack was convicted of accepting bribes from athletes suspected of doping to cover up test results and letting them continue competing, including in the 2012 London Olympics.
He was sentenced to four years in prison, two of them suspended.
Diack's lawyers said he would be appealing against the judgement, which they called "unfair and inhumane."
Diack, who was also given a maximum fine of €500,000 (£456,928), was investigated by French authorities for four years over claims he took payments of more than €3 million to cover up cheating.
The judge said his actions had "undermined the values of athletics and the fight against doping".
Diack has been under house arrest in Paris since November 2015.
Previously one of the most influential figures in world sport, Diack was president of the International Association of Athletics' Federations (IAAF) – now World Athletics – for 16 years until he was replaced by Britain's Lord Coe in August 2015.
Diack's son Papa Massata Diack – who was banned for life from athletics in 2016 – was sentenced to five years in prison and a given a €1-million fine (£913,850).