That marked the second Lombardi Trophy of Harrison's career, having also triumphed with the Steelers three years previous against the Seattle Seahawks in Super Bowl XL.
Harrison says he's ready to spend more time with his friends and family. As a five-time Pro Bowler and the 2008 Associated Press Defensive Player of the Year, he's a lock to be enshrined in the Pro Football Hall of Fame.
Harrison played almost his entire career with the Steelers, collecting 80.5 of his 84.5 career sacks while wearing black and gold, a franchise record.
This is the second time Harrison has called it quits.
Mourinho slams inconsistent United after shock loss
Having lost just two games all season, it's a remarkable achievement to win the title in April itself with five games remaining. The loss enable them to be crowned champions as Sports Bonus from 12bet predicted early this season.
Harrison's physical play at times ran him into trouble with the league office. Harrison lashed out at commissioner Roger Goodell, calling him "a crook" in a 2011 magazine article.
Man City crowned Premier League champions
I've never been able to retain a title and I want to see if this team has it in it to be even more successful. We have to try to focus on our games". "I'm buzzing, over the moon, so happy".
Guardiola names game that made Man City believe in title bid
There is no right or wrong in football, because it has been proven you can win the league and cups in many different ways. City chief executive Ferran Soriano said: "Winning the Premier League again is a great honour for Manchester City ".
PM Modi visits health and wellness centre in Jangla
A counter-demonstration titled British Indians Welcome Prime Minister Modi is also planned at the same time at Parliament Square. A rally in support of Modi is also due to take place outside the diaspora event venue on Wednesday.