Three weeks ago, the 49ers were considered NFL's best at 5-0.
The 49ers have now lost three consecutive games, dropping their record to 5-3.
Bengals' Joe Burrow had an outstanding performance with 28 of 32 passes completed, three touchdowns, and no interceptions.
Ja'Marr Chase was Burrow's top target with 10 catches for 100 yards and a touchdown.
Joe Mixon contributed with 15 carries for 89 yards and a game-clinching touchdown for the Bengals.
George Kittle and Brandon Aiyuk both had over 100 receiving yards for the 49ers.
Despite their rough start, the Bengals improved to a 4-3 record, while the 49ers looked less impressive than three weeks ago.