The 22-year-old's contract at Blackburn Rovers expired at the end of last season and was free to discuss his future with any potential suitors.
QPR have beaten off stiff competition to sign the winger, who netted seven times for Rovers last season.
"Junior is a player I know very well," manager Mark Hughes told the club’s official website.
"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, Hughes' seventh summer signing, added: "QPR are a club that are certainly heading in the right direction. 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.
"The manager obviously played a big part in my decision to come here. He's someone I know well here having worked with him and his backroom team at Blackburn.
"I signed my first professional contract under him and I will always remember that and be grateful to him. Working under such a great manager will help me to develop my skills and further my career.
"I have not achieved anything yet. I am still at the early stages of my career and have a lot to learn. As a young player it is important to stay humble, and listen to your family and friends.
"I'm really happy to be here and can't wait to pull on the Rangers shirt."
