The 73-year-old South Korean replaces Youssoupha Ndiaye of Senegal as head of the IOC's ethics commission.
Ban takes the job at a time as the IOC comes under fire again after its Brazilian honorary member Carlos Nuzman was arrested in connection with a corruption probe around the awarding of the 2016 Rio Games.
French authorities are also looking into payments around Tokyo's election to host the 2020 Games, and the dealing with the Russian doping affair has also tarnished the image of the IOC.
The IOC and its president Thomas Bach said they were awaiting details from the Brazilian police on the issue before launching an ethics probe.
Ban was UN secretary-general between 2007 and 2016 and has had a close relationship with the IOC which in 2009 was granted permanent observer status at the UN.
He has delivered a keynote address at a past IOC Session and also been Olympic torch bearer.
"I believe that ethics is essential to the success of any organisation," Ban told the IOC Session.
"That is why I did everything possible to strengthen the culture of ethics at the United Nations. I promoted transparency and accountability in every way I could, and I tried to lead by example."
Photo: AFP / UN / Rick Bajornas Former UN Secretary General Ban Ki-moon