Koetter's status had been tenuous due to this season's disappointing performance. The Buccaneers are 4-11 entering Sunday's finale against the New Orleans Saints.
Tampa Bay went 9-7 in 2016 in Koetter's first season at the helm.
But quarterback Jameis Winston regressed in 2017 and also missed time with a shoulder injury. The Buccaneers are currently mired in their second five-game losing streak of the season.
Recent speculation had former Bucs coach Jon Gruden (2002-08) as a possible candidate if Koetter was relieved of his duties.