The versatile former South Korea international, 31, moves to Loftus Road for a reported initial fee of £2 million which is believed could rise to as much as £5m based on appearances and success.
The announcement was made at an afternoon press conference on Monday, at which the club are also expected to reveal preliminary talks over more than one potential site for a new stadium in or close to the White City area of West London.
"On a football level Ji is someone I have admired from afar for years," QPR manager Mark Hughes said of his sixth summer signing. "He wore the red shirt of Manchester United in a distinguished way and he will bring a great deal to the team, I can't wait to start working with him.
"Ji is a motivated guy, he feels there is success ahead of him, he ticks every box. It's a very exciting time for QPR.
"I said at the end of the season we didn't want to be in the same position next season and I stand by that, we are in a better place now."
Former PSV midfielder Park, who rose to prominence at the 2002 World Cup, has penned a two-year deal and lines up alongside former United team-mate Fabio Da Silva, who has joined on loan from Old Trafford.
Park has played over 200 matches for Manchester United, scoring 27 goals over a seven-year period following his £4m move from PSV in 2005.
The Korean, who won four Premier League titles and a Champions League during his time at United, is understood to have been swayed to join QPR by their ambitious plans.
"I know QPR want to improve and have ambitions to reach the next level and I will try my best to help," he said.
"There were other offers but nothing compared to QPR."
QPR have also signed Andrew Johnson, Ryan Nelsen and Robert Green this summer, as well as making Samba Diakite's loan move permanent, and are expected to make more signings.