The deal is worth $10 million, including a $2.1 million signing bonus, a league source told ESPN.com's John Clayton.
The Steelers have a policy of not negotiating with players once a season begins, so Hartwig's deal -- which runs through 2012 -- is likely the last until the offseason.
Hartwig enters his second season as the Steelers' center and eighth in the NFL. He has started 79 career games, including some with Tennessee and Carolina. Hartwig was a sixth-round pick by Tennessee in 2002 and signed with Pittsburgh after being cut by Carolina following the 2007 season.
Information from The Associated Press was used in this report.