In an Instagram post on his account that features photos of his children, he writes: "I've missed way too much for way too long…and I'm done. Many thanks to my family, coaches, the fans, and everyone who played a role in my football life."
Harrison, who signed with the New England Patriots?on Dec. 26 after he was released by the Pittsburgh Steelers, had said in the week leading up to Super Bowl LII that he hoped to play another season or two.
Harrison ended up being used a lot more than many anticipated by New England. After having played just 40 snaps over 14 games with the Steelers last season, Harrison was on the field for 69 snaps in Super Bowl LII.
That reflected, in part, how the Patriots had a dire need for at their end-of-the-line linebacker position in 2017.
Harrison had five tackles and two sacks in the team's regular-season finale — his first game with the Patriots — and totaled six tackles and a quarterback hit in three playoff games.
Bill Belichick credited him for his professional approach in the playoffs.
As for Harrison's eventual retirement, Harrison previously told ESPN: "I told my son [James III] he could play contact football when I stop, so I'm going to have to give it up in the near future, so he can start getting adjusted to playing. I don't want him to wait too long."
Harrison had a franchise-record 80.5 sacks in 14 seasons with the Steelers.
In August 2014, Harrison announced his retirement, but by September he was back with Pittsburgh.?
ESPN's Mike Reiss contributed to this report.