The 49-year-old began his second spell in charge in February 2019 when Brendan Rodgers joined Leicester City but has now resigned from his position.
Lennon led Celtic to a historic Treble in 2019/20 but results have been disappointing this season and they currently trail Rangers by 18 points in the Scottish Premiership title race.
Celtic Chief Executive Peter Lawwell told the club website: “I would like to pay tribute to Neil for all he has done for the club in his second spell, delivering our eighth and ninth successive league titles, the quadruple Treble and winning the last five available domestic trophies.
“While this season has not progressed as we would have liked, it cannot diminish the character or integrity of a man who has given the club so much.
"I would like to thank Neil for his work as our manager and I wish him and his family good health and continued success in everything they do.”
John Kennedy will take charge of the team on an interim basis, with the club's next match against Aberdeen at Celtic Park on Saturday.