The Pittsburgh Post-Gazette said the agreement is worth more than $51 million.
Harrison is coming off his best season as a pro. He set a Steelers record with 16 sacks, was second on the team with 101 tackles and led the NFL with seven forced fumbles.
But he probably is best known for a play in Super Bowl XLIII in which he intercepted a Kurt Warner pass and returned it 100 yards for a touchdown as the first half ended. Pittsburgh won the game 27-23. It is the longest play in Super Bowl history.
"The play in the Super Bowl just let other people, who might not see it all the time, realize what James Harrison has been doing all along since getting his chance," Harrison's agent, Bill Parise, told the Post-Gazette.