Johnson, a two-time Pro Bowl player, would be used as a backup and an insurance policy in the final weeks of the regular season, Bengals head coach Marvin Lewis told reporters.
Johnson, who turns 30 this week, was released by Kansas City last Monday after six-plus seasons with the team. He had rushed 132 times for 358 yards with no touchdowns in seven games with Kansas City this season, while in his 75-game career has gained 6,015 yards and scored 55 touchdowns.
He received a two-week suspension from the club this year after posting demeaning comments on the Internet about Chiefs head coach Todd Haley and for using anti-gay slurs.
The Bengals' running back situation became unsettled after Cedric Benson sustained a hip injury during Sunday's 18-12 win over Pittsburgh, a victory that lifted Cincinnati into sole possession of first place in the AFC North at 7-2.
The Bengals waived defensive tackle Orien Harris to make room for Johnson on the roster.