The Broncos needed a reinforcement in the secondary after second-year starter Quinton Carter injured his right knee and hamstring in a nasty fall inside the team's practice bubble July 27.
Leonhard is a seven-year veteran who spent the past three seasons emerging as a fan favorite with the New York Jets, who didn't sign him in the offseason after he underwent surgery in December to repair a tendon in his right knee.
The injury generally takes between eight and 10 months to heal.
After starring at Wisconsin, Leonhard played three seasons in Buffalo and one in Baltimore. He was a three-year starter in New York, where he was noted for his hard-hitting style despite his under-sized 5-foot-8, 190-pound frame.