Tom Pelissero of NFL Network first reported the news.
Lewis leaves Cincinnati having compiled a 131-122-3 record over 16 seasons.
This past year, the Bengals finished last in the AFC North with a 6-10 record. They've now missed the playoffs for three straight seasons.
To some extent, Lewis is a victim of his own success. There was once a time when a three-year postseason drought was nothing for the Bengals. Between 1983 and 2004, they had only two playoff appearances.
But Lewis ushered in a new era for the Bengals and raised the bar for what was expected. Cincinnati reached the postseason in seven of his 16 years on the sideline and won four division titles.
As much as he did to help the Bengals achieve a level of respectability it hadn't enjoyed in decades, one could argue his departure is overdue.
The Bengals never advanced past the AFC Wild Card Round under his watch, which generated a level of frustration among the fanbase.
ESPN's Stephen A. Smith summed up the general reaction when news broke of Lewis' extension with Cincinnati last January:
Many originally thought Lewis' deal guaranteed him two more years, but NFL Network's Ian Rapoport reported in December the second year was a team option.
For the Bengals, firing Lewis represents a good start toward the beginning of a new era. Following through with the right replacement is just as important.
Before Lewis arrived, the Bengals were one of the NFL's biggest laughing stocks. Much of the problems started with ownership, which made bad personnel decisions and earned a reputation for being extremely frugal.
If Cincinnati misfires with its choice to succeed Lewis, those days might be back once again.
Many of the Bengals' key veterans are either banged up or on the wrong end of the aging curve. Andy Dalton and A.J. Green are both in their 30s and coming off season-ending injuries. Vontaze Burfict suffered the seventh documented concussion of his NFL career in 2018. Geno Atkins reached his fifth straight Pro Bowl but also turns 31 in March.
Then there are the high draft picks who haven't panned out (Cedric Ogbuehi, John Ross, Darqueze Dennard, Dre Kirkpatrick and Jeremy Hill) and set the Bengals back.
Because the Bengals are headed for a rebuild, making a coaching change is sensible. But things started turning around when ownership empowered Lewis to make roster decisions.
That could change with a new head coach who hasn't built any equity with the players or those in a position of power.