Nottingham Forest are looking for an 'impact' player on loan, but are still refusing to comment on Jack Grealish links.
Sky Sports understands former Wolves midfielder Jamie O'Hara could be in line for a move to America with Chicago Fire.
Press Association Sport understands Cardiff agreed a fee of £8million to secure his services on a four-year deal.
Caulker told Cardiff City Player: "First and foremost I want to say how happy I am to have signed with Cardiff City Football Club. I'm delighted to be here and excited for this fresh challenge.
"I spoke to the gaffer yesterday, liked what he had to say and am excited about the plans he has for the football club.
"You get the sense of excitement from everybody at the club in the build-up to the season's kick-off at West Ham and I'm really looking forward to being part of it."
Caulker is Malky Mackay's fourth signing of the summer and will provide stern competition for Mark Hudson and Ben Turner at the heart of the Bluebirds defence.
Mackay has already added Danish striker Andreas Cornelius, Derby full-back John Brayford and Brentford goalkeeper Simon Moore to his squad ahead of Cardiff's maiden Barclays Premier League campaign.
The acquisition represents something of a coup for the Welsh club, as Caulker had established himself at White Hart Lane, making 28 appearances in all competitions for Spurs last term and catching the eye of England boss Roy Hodgson.
He made his debut for the Three Lions last November and was on the scoresheet in the 4-2 friendly defeat against Sweden in Solna.
Caulker's switch to Cardiff City Stadium is also likely to raise the temperature at this season's south Wales derby fixtures as the centre-half enjoyed an excellent stint on loan at Swansea during the 2011/12 campaign, featuring on 26 occasions for the Swans.