Flacco completed 21-of-29 passes and Ray Rice gained 93 total yards, including two rushing scores, as Baltimore (1-0) put the game away with 27 unanswered points.
The Ravens won their 11th straight home game on a night when they honored former owner Art Modell, who died Thursday at age 87.
Modell, who controversially moved the team from Cleveland to Baltimore in 1996, was instrumental in instituting Monday night football in the early 1970s.
The Ravens' Ed Reed executed his seventh interception return for a touchdown.
Andy Dalton connected on 22-of-37 pass attempts for 221 yards with an interception and a lost fumble for the Bengals (0-1).