The 31-year-old South Korea international brought his successful seven-year spell at Old Trafford to an end by joining Rangers for an undisclosed fee believed to be upwards of £2million.
"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.
"QPR want to be a bigger club, they want to improve and it's a great challenge. 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.
"I am joining a club that is very much on an upward curve, a club that has a very bright future.
"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."