The 24-year-old has signed a four-year contract at Loftus Road and will now compete with Rob Green for a starting spot in goal for Rangers.
McCarthy, who made three appearances for England's Under-21s, will wear the No 12 shirt for QPR.
He told Sky Sports News HQ: "The Premier League's where you want to be and I'm over the moon.
"Everyone knows how good Rob Green is, and I can learn a lot from him, but I don't want to be a No 2 keeper. I want to play games.
"QPR are a massive club and they've got massive players there now. I'm looking forward to training and getting to know the squad."
McCarthy earned a call-up to the England squad in the summer of 2013, and he said: "The main thing for me is to get playing and give myself a chance of being in that position again."
Rangers manager Harry Redknapp said: "I am delighted to bring Alex in.
"He's a great young goalkeeper with a great future in front of him. I think he's got the potential to be a future England goalkeeper one day, so to have him here at QPR is fantastic.
"We've got Rob Green and Brian Murphy, and Alex's arrival here helps to further strengthen the goalkeeping department.
"I want to have competition for places in all areas of the pitch, and you can't have too much quality in your squad.
"He is a great signing for the club, and for the future of QPR."