The move came after the coaching staff decided Ryan Kalil would replace Hartwig as the starter in 2008.
"We had decided to go in another direction at center and Justin would have been able to come in and compete for the starting spot at right guard," general manager Marty Hurney said. "But we decided probably the right thing to do for Justin was make this move now and see if he can get with a team that would consider him as a starter at center."
Hartwig joins left guard Mike Wahle as the second starter on the line to be let go since Carolina finished a disappointing 7-9 season.
Two years ago, Carolina lured Hartwig away from Tennessee with a five-year, $17 million deal. But Hartwig wasn't the dominant player the Panthers had hoped. He missed most of the 2006 season with a groin injury, and the Panthers drafted Kalil in the second round last year.
While Hartwig started 15 games last season before missing the finale with a knee injury, the Panthers planned on Kalil being the long-term answer at center.
"There are things like this that happen all the time in free agency. But this certainly says nothing negative about Justin," Hurney said. "He did everything we asked since he came in here."
The Panthers have identified the offensive line as one of their top offseason priorities.
Carolina placed the franchise tag on right tackle Jordan Gross and re-signed left tackle
Travelle Wharton, who could move to left guard next season. The Panthers also signed free-agent guard Keydrick Vincent, who could end up as the starting right guard.
Carolina also could pick up a tackle with the 13th pick in the draft.
Hartwig is the latest veteran to be jettisoned in an active offseason for the Panthers, who have missed the playoffs for two straight years. The Panthers have also released starting running back DeShaun Foster, linebacker Dan Morgan and quarterback David Carr.