The industrious 31-year-old midfielder spent seven years at Old Trafford after arriving from Dutch club PSV Eindhoven, scoring 27 goals in 203 appearances for United.
Park -- capped over 100 times and the only Asian to score in three successive World Cup finals tournaments (2002/06/10) -- helped United win four Premier League titles, three League Cups and the 2008 Champions League but couldn't quite nail down a first-choice spot in the Red Devils' midfield.
QPR only avoided relegation on the final day of last season and Park said it had been tough to leave Manchester to join the west London side.
"It was a very difficult decision, it's hard to leave such a big club, but QPR showed me what they are trying to be," he said.
"It is time for a new challenge."
Park, who signed for an undisclosed fee believed to be more than ?2 million, reportedly received offers from leading clubs in England, Europe and Asia.
However, he decided to join a QPR side owned by ambitious Malaysian businessman Tony Fernandes and managed by former United striker Mark Hughes.
"QPR want to be a bigger club, they want to improve and it's a great challenge," Park said. "It was a difficult decision but they have great players and they are hungry to get to the next level. It's not about the money.
"The manager Mark Hughes has a proven track record at Premier League level and I am looking forward to working with him and the rest of the squad to help take the club to the next level."
Park is Rangers' sixth pre-season signing, joining Ryan Nelsen, Andrew Johnson, Robert Green, Samba Diakite and his former United team-mate Fabio at Loftus Road.
"On a football level Ji is someone I have admired from afar for years," said Hughes.
"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.
"He's very adept in a number of positions. What he brings to the team is technical ability, a huge work ethic and a winning mentality.
"He understands what it takes to win trophies and to win matches, and I want to take that from Ji and pass it to the rest of the team.
"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."
Meanwhile United manager Sir Alex Ferguson was sorry to see Park go, saying: "He is the ultimate professional and such a nice lad. He never let us down on the big occasions.
"Unfortunately, I just couldn't give him the number of games he wanted."
Source: AFP
Source: AFP