Park joined the Reds from PSV Eindhoven in 2005, played 205 times and collected eight major honours, including four league titles and the FIFA Club World Cup.
"My time at United will last in my heart for the rest of my life," he told ManUtd.com.
"It’s been a great privilege to be part of such a great team, to have won so much and to have played with special teammates and for the greatest manager in the game.
"I would like to thank everybody at the club who give their best every single day to put every player in the best condition to enjoy their football and achieve success."
Park was a firm fans' favourite at Old Trafford and his name was often sung loudly by United supporters, even when he wasn't on the pitch.
“The fans have been fantastic to me and I will always remember them with great affection," he said. "Now, I’m looking forward to giving everything for a new challenge at QPR, where I will bring all my ambition and spirit I have gathered at United. [Those qualities] will be with me forever."