Leonhard earlier announced the deal on his Twitter account, writing: "It's never a bad day to sign an NFL contract. I'm proud to be a Denver Bronco. Time for work."
The seven-year NFL veteran received a one-year contract, a source with knowledge of the deal told NFL.com and NFL Network's Ian Rapoport.
Leonhard, 29, is expected to fill the hole left by strong safety Brian Dawkins, who retired after the 2011 season. However, Leonhard is coming off December surgery to repair a torn tendon in his right knee.
Leonhard has intercepted a pass in each of his last five seasons. He has six interceptions and one touchdown in his career. He also has returned punts and kickoffs throughout his career.