The nippy wideman left his loan spell at Cardiff City yesterday morning after rejecting their offer of a three and a half year deal and moved to Loftus Rd instead.
He became the thrd permanent signing made yesterday after Heidar Helguson and Gary Borrowdale.
"I'm absolutely thrilled to be here," Routledge told the club's official website. "I am delighted to have sealed the move."
"QPR are a club with huge ambitions and as soon as they showed interest in me I was determined to speak to them."
Routledge added: "The manager has a great vision about where he wants to take the club and he sees me playing a big part in that.
"It's nice coming back to London, but the geography didn't play a big part in my decision- the reason I am here is because the manager showed an unbelievable desire to get me to QPR."