The Latics, who dropped into the fourth tier on the final day of last season, have promised to appoint a new head coach "as soon as possible".
"Oldham Athletic can confirm that the contract of manager Richie Wellens has been terminated with immediate effect," read a statement on the club's official website. "We would like to thank Richie for all his efforts.
"Whilst he was unable to guide us to League One safety after being appointed in October, it certainly wasn't due to a lack of professionalism and dedication on his part and we wish him every success for the future.
"The club will look to appoint a new manager as soon as possible and will be making no further comment at this time."