PA PhotosThe Canadian winger becomes the Loftus Road club's seventh signing of the summer following the arrivals of Rob Green, Ryan Nelsen, Fabio Da Silva, Ji-Sung Park, Samba Diakite and Andy Johnson.
"Junior is a player I know very well," said QPR manager Mark Hughes.
"He was just a young boy trying to make his way in the game when I had him at Blackburn. It was difficult for him then because of work permit issues, but I had him training with the First Team very early on because his talent was there for all to see.
"We knew then he was going to have a great future and he's certainly showed that talent over the last few seasons at Ewood Park.
"He's shown he's a player of top quality and a player that can makes things happen, both on his own and as a team player.
"He's certainly a player that will enhance what we've got in the group and I am delighted he's chosen to come to us."
Hoilett scored seven goals for Blackburn last term, but it was not enough to halt their relegation to the Championship. Rovers were apparently keen to let a tribunal decide a fee for the 22-year-old with a figure of £6 million now being touted as a potential figure.
Blackburn are chasing several million pounds of compensation under FIFA rules having developed Hoilett since he joined their academy as a teenager.
"QPR are a club that are certainly heading in the right direction," said Hoilett. "The ambition to take this club to the next level was something I really wanted to be part of.
"The manager and the owners have huge ambitions to achieve things here and this is the perfect place for me to further my career."
