Earlier on Tuesday the 22-year-old was undergoing a medical ahead of a move to Loftus Road and club chairman Tony Fernandes later confirmed the transfer has been finalised.
I would like to announce the signing of Stephen Caulker to Queen's Park Rangers. Welcome. pic.twitter.com/3c6QvziDbK— Tony Fernandes (@tonyfernandes) July 22, 2014
Caulker was also the subject of significant interest from Southampton but has ultimately taken the opportunity to reunite with former boss Harry Redknapp and partner in defence with fellow summer signing Rio Ferdinand.
The former Tottenham youngster signed for Cardiff last summer for €10m after impressing on loan at Welsh rivals Swansea City, making 38 Premier League appearances as the Bluebirds were relegated.
On the announcement of the deal, Caulker told QPR's official website: "Harry [Redknapp] 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 added: "Steven’s a top player, so we’re delighted to have him at QPR.
"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.
"I’m sure he will come in and show exactly what he’s capable of – he’s got everything to be a top, top centre-half, so I couldn’t be more pleased to have him."