Terms were not disclosed, but NFL Network reported the contract includes a fully guaranteed $7 million base salary and up to $2 million more in incentives.
Campbell, 36, was released by the Baltimore Ravens on March 13 in a cost-cutting move. The Ravens, who cleared $6.5 million in cap space, had been hoping to re-sign Campbell on a lesser deal.
Instead, Campbell will take his six Pro Bowls and 99 career sacks to a team that has vastly improved its defense in free agency. The Falcons also added safety Jessie Bates III, defensive tackle David Onyemata and linebacker Kaden Elliss.