Financial terms weren't disclosed.
Although Hardin hadn't signed a contract prior to the start of the club's three-day rookie minicamp last week, he participated in drills after signing an injury waiver.
The sides struggled to come to an agreement on a deal in part because of the wide-ranging amounts of bonus money being paid to third-round picks, and not many contracts for third-rounders had been done around the league, thus making a bonus difficult to slot.
Deals for the first-rounders and players taken between the fourth and seventh rounds, meanwhile, fell into slots, making them easier to complete.
A former standout cornerback at Oregon State, Hardin missed the entire 2011 season with a shoulder fracture, and will look to make the transition to safety for the Bears.
"Mentally, it's a little difficult," Hardin said. "Physically though, I have all the aspects to be a good safety. Now it's just learning a new position, taking in the coaching and learning the different schemes and coverages. It's been going great. I've definitely been learning a lot."
Hardin (6-foot-3, 217 pounds) posted 63 tackles at Oregon State in 2010, and looks to work his way into a rotation that includes 2011 rookie Chris Conte, in addition to Major Wright, Craig Steltz and Anthony Walters.
"In this system, the No. 1 thing the Bears pride themselves on is toughness," Hardin said. "I know being a physical safety, whether it's coming down hitting, being in the box, making secure tackles and takeaway, on this team we want to score every play. There's going to be a learning curve there. But I do have to learn as fast as I can if I want to play."