The deal is for one year, a source told ESPN's Vaughn McClure.
The 27-year-old Bethel joins guard Brandon Fusco and tight end Logan Paulsen as Atlanta's top free-agent additions. The Falcons have focused on re-signing their own players while hoping to work out an extension for quarterback Matt Ryan, who can be a free agent after the 2018 season.
Bethel leaves the Cardinals after three Pro Bowl appearances and two first-team All-Pro nods as a special teamer, but he didn't pan out as a defensive back.
He had a combined four interceptions the past three seasons with the Cardinals and returned three for touchdowns. Bethel has avoided serious injuries with Arizona, playing in all 16 games in each of his six seasons.
A sixth-round pick in 2012 of Presbyterian College, Bethel lost his starting cornerback job last season after Week 6 and didn't play more than 14 defensive snaps the rest of the season.
He signed a three-year extension in December 2015 worth $15 million guaranteed but restructured it last offseason.
ESPN's Josh Weinfuss and The Associated Press contributed to this report.