Harrison's agent, Bill Parise, said the parting is amicable, but Harrison was frustrated with a lesser role in the defense and wants to continue playing.
"There was no animosity or bad feelings. It's just the business of the NFL," Parise told ESPN's Josina Anderson. "I believe he still wants to play. We'll have to wait and see what happens with waivers."
Harrison was the oldest defensive player in the league before he was cut. He and Minnesota Vikings cornerback Terence Newman, who is four months younger than Harrison, were the only 39-year-old defensive players to play in a game this season, according to ESPN Stats & Info.
Harrison thanked Steelers fans on Instagram for their "years of love & support."
Harrison had 80.5 sacks in a Pittsburgh uniform, including at least five sacks in each of the past three regular seasons, despite playing a part-time role.
He came up big in the past two playoff runs, with multiple sacks, but his role decreased this season as the Steelers transitioned to more pass-coverage work for outside linebackers Bud Dupree and rookie T.J. Watt.
Harrison played 40 snaps through 14 games this season, compared with 587 last season.
He signed a two-year deal in March. He keeps himself in excellent shape -- he posted an Instagram video of his workout Saturday morning -- and could help a team that needs a freakishly strong pass-rusher off the edge.
Said Parise: "We'll see what Santa brings us."
Harrison signed with Pittsburgh as an undrafted rookie free agent on April 22, 2002. He has played 177 of his 192 career games for the Steelers, including 107 starts. Over 14 NFL seasons, he has 788 tackles, 82.5 sacks (two with Cincinnati) and eight interceptions.
He was the 2008 AP Defensive Player of the Year and a five-time Pro Bowl selection. His 100-yard interception return for a touchdown in Super Bowl XLIII was the longest in Super Bowl history, according to ESPN Stats & Information.