"This is a real coup for QPR," manager Mark Hughes, himself a former United player, told the club website.
The club did not disclose the fee for the 31-year-old, who said he was looking forward to help take the club to a new level, but media reports said the agreement could be worth five million pounds.
Park, who made more than 200 appearances for United since joining from PSV Eindhoven in 2005, has a reputation as a big game player able to relentlessly break up opposition attacks and send his own team-mates on the offensive.
He was the first Asian to compete in a Champions League final in 2009, and won four Premier League titles at United. The former captain of his country, he made 100 appearances for South Korea between 2000 and 2011.
"His record speaks for itself," said Hughes. "He has been a big player for Manchester United and played a big part in their success in recent years.
"He was always picked for the big matches because they could rely on him and we are going to reap all those qualities."
QPR's Malaysian chairman Tony Fernandes, whose AirAsia airline sees South Korea as a growing market, said he and Hughes had worked hard to make the deal happen.
"I am the ultimate dreamer," he declared. "What started as a little idea is now a reality and it shows that people are beginning to buy into our story."
Park is the second United player to move to QPR this month following young defender Fabio da Silva who has joined on a season-long loan deal.
Fabio, who can play at full-back or in midfield, has played 53 times for United, but has rarely been a first-choice starter.
QPR are seeking to establish themselves in the Premier League after securing their safety on the last day of the season following their promotion in 2011.
Fernandes told Reuters at the weekend that QPR could sign three or four more players before the start of the Premier League season on top of those already announced.
