It was a mutual decision, according to The MMQB's Albert Breer.
Cooter became the Lions' quarterbacks coach in 2014, and he was promoted to offensive coordinator in 2015 after Joe Lombardi was fired.
The 34-year-old is drawing interest from other teams, according to Breer.
Cooter's contract with the Lions was set to expire at the end of the season, and Detroit was widely expected to move on from him after the team's scoring offense ranked 25th in 2018, averaging 20.3 points per game.
The Lions have yet to announce Cooter's replacement.