Lewis' contract expired at the end of the season, and the announcement came after he and team officials had met several times.
Lewis has registered a 60-67-1 regular season record during his tenure as head coach.
The Bengals have posted a pair of winning seasons during his reign -- 2005 (11-5) and 2009 (10-6). They won the AFC North in each of those years but exited the playoffs in the opening round both times.
Reports indicated Lewis is seeking changes in how the Cincinnati organization is run.