Harrison was the NFL's Defensive Player of the Year last season and had a 100-yard interception return before the half in the Steelers' Super Bowl victory over the Arizona Cardinals. He was scheduled to make $1.4 million this season, the final year of his contract.
Included in the contract, which will be officially announced Tuesday, is about $20 million in bonuses.
Harrison rose from being an undrafted player cut a few times into one of the best defensive players in professional football. The 30-year-old Harrison had 16 sacks in 15 regular-season games last year and he had 8½ sacks in 2007. He took over a starting job after the departure of Joey Porter following the 2006 season and has 199 tackles during his two seasons as a starter.
The contract extension for Harrison was considered one of the main goals for the Steelers during the offseason. The team wanted to reward him for last season along with preventing him from hitting the free-agent market in 2010.