The 23-year-old has scored six goals in 25 games since initially joining Wycombe on loan in July and has now signed a three-and-a-half-year deal.
Harriman, who is equally capable in defence or midfield, made just three first-team appearances for the R's.
"He's got everything you want from a professional," boss Gareth Ainsworth told the club website.
"He gives his all, he's got bags of ability and he's a great character to have around the club.
"To be able to keep him here is a fantastic piece of business and I'm grateful for the support of the board who have enabled me to make Michael a permanent signing."
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League Two side Wycombe Wanderers have signed utility man Michael Harriman from QPR for an undisclosed fee.