The five-year veteran, originally an undrafted free agent from Kent State, capped a huge 2008 season by returning a pass interception 100 yards for a touchdown on the last play of the first half in Pittsburgh's 27-23 XLIII Super Bowl win on Feb. 1 against the Arizona Cardinals.
It was the longest play in Super Bowl history.
"It' been a very open process, and it's going about the way you would expect," Harrison's agent, Bill Parise, said about the initial contract talks.
Harrison, 30, compiled totals of 101 tackles, 16 sacks, and seven forced fumbles, helping the Steelers earn a record sixth Super Bowl title.
Harrison, who made a $1.2 million salary in 2008, likely will get a salary upgrade next season.
He is one of several players who will be free agents in 2010, including receiver Hines Ward, tight end Heath Miller and defensive tackle Casey Hampton.