Lennon's last game in charge was Sunday's 1-0 defeat to Ross County, which left Celtic's hopes of a 10th consecutive league title further in tatters.
Celtic completed a quadruple Treble in December by winning the 2019-20 Scottish Cup but Lennon had come under increasing scrutiny after falling well behind Rangers this season, who need just seven points from their last eight games to secure the league title.
"We have experienced a difficult season due to so many factors and it is very frustrating and disappointing that we have not been able to hit the same heights as we did previously," Lennon said in a statement.
"I have worked as hard as ever to try and turn things around, but unfortunately we have not managed to get the kind of run going that we have needed."
Assistant manager John Kennedy will take interim charge of Celtic, who host fourth-placed Aberdeen on Saturday.