The centre-back was linked with several clubs following Cardiff's relegation from the top flight last term, but has opted to return to London and head to Loftus Road.
Caulker, who only joined Cardiff from Tottenham a year ago, is relishing the prospect of working with Harry Redknapp again, having played under the 67-year-old at White Hart Lane.
And the opportunity to play alongside former England defender Rio Ferdinand, who moved to QPR as a free agent last week, was also a big factor in the 22-year-old's decision.
The England international told the club's official website: "I am very excited to be here.
"It was important for me to be back playing in the Premier League, and Harry Redknapp has given me the opportunity to be at this fantastic club.
"Harry was a big factor for me, having worked with him at Tottenham. He was very keen for me to join and that was a big plus, knowing the manager wants you.
"Being able to play with Rio was also a big draw. He has been a role model of mine for many years, along with Ledley King. To have the opportunity to play alongside him was something I felt I couldn’t miss out on.
"Our first aim this season has to be to get to 40 points, to make sure we stay up. That's our first goal."
Redknapp believes QPR have pulled off something of a transfer coup in landing Caulker, who played every minute of Cardiff's Premier League season in 2013-14.
He said: "I know the ability he has and I think it's an important signing for the club – for the here and now, and for the future.
"I had him at Tottenham and I was very surprised when they decided to let him go last summer, because I thought he had a big future there.
"He's young; he's a good age; he's already a fine player but he has so much more potential – it's a really excellent signing for the club.
"When good players like Steven become available, who can come in and have a real impact, you’ve got to try and get them."
