Lewis was entering the final year of a previous 2-year contract extension. Financial terms weren't released.
The Bengals are 69-74-1 in the regular season under Lewis and twice won the AFC North. The team, however, is 0-3 in the playoffs with Lewis as coach. Cincinnati was 9-7 last season, losing to Houston in the first round of the playoffs.
"Coming off a playoff finish last year, with a solid coaching staff and good young players, continuity at the top gives us our best chance to do well," Bengals President Mike Brown said in a statement.
"We are excited about the team's prospects and happy to have completed this process with Marvin before the 2012 preseason schedule begins."