Bell's deal is for three years and $18 million, a source told ESPN's Adam Schefter.
Bell, 25, has shown steady improvement since being drafted in the second round out of Ohio State in 2016.
The 5-foot-11, 205-pounder was an every-down player for the Saints when healthy in 2019. He set a career high with 89 tackles in just 13 games played, and he reeled in his first career interception. He also led the NFL with five fumble recoveries in the regular season -- then added a sixth in the Saints' playoff loss to the Minnesota Vikings.
As valuable as Bell was to the Saints, he became somewhat expendable for the salary-cap-strapped team because of the emergence of 2019 rookie C.J. Gardner-Johnson.
Bell has settled in as a strong safety in recent years, though he showed the versatility to play both spots early in his career. He has nine career sacks and seven career forced fumbles, including the playoffs.
Bell gives the Bengals another experienced safety, providing defensive coordinator Lou Anarumo more flexibility with his personnel.
In 2019, the Bengals used a strong safety closer to the line of scrimmage as the team struggled with its depth at linebacker. This left free safety Jessie Bates centered at the top of the defensive formation for most of the season. With Bell in the fold, the Bengals could opt to play him and Bates at the traditional safety spots while using Shawn Williams in the role he played in 2019.
Cincinnati is looking to improve a defense that surrendered more yards per play than any other unit in the NFL last season and continued its free-agency overhaul Wednesday with commitments from Bell and former Tennessee Titans cornerback LeShaun Sims. The Bengals have now added six new defenders on deals that are worth at least $115 million combined.
ESPN's Mike Triplett and Ben Baby contributed to this report.