The 33-year-old scored 27 goals in 205 Reds appearances and was a huge fans' favourite during his seven-season stay at Old Trafford, where he won four Premier League titles, three League Cups, one UEFA Champions League and the FIFA Club World Cup.
"I've been mulling retirement since February, and I've concluded that my knee won't hold up for another season,” Park explained at the JS Football Center in Seoul, where his own charity foundation is based. “I have no regrets about my career.
“I do think about what might have been if I hadn't been injured, but I have no feelings of disappointment or sorrow as I leave the sport.”
Park finished as a QPR player but spent his last season on loan at PSV Eindhoven, his first European club, where he made 27 appearances and scored two goals to help his team-mates finish fourth in the Dutch Eredivisie.
Everyone at Manchester United sends their best wishes to Ji.