The Bluebirds had already agreed a fee for the 25-year-old, believed to be around the £1.5million mark, and the formalities of the deal have been quickly wrapped up.
Brayford told Cardiff City Player: "When I heard Cardiff were interested in signing me then I was obviously very interested, especially at the prospect of playing in the Premier League and testing myself at the highest level.
"Derby will always be a special club to me, but this was an opportunity I couldn't ignore."
He made 117 appearances in total for the Rams following his move to Pride Park from Crewe Alexandra in May 2010.
- Sports & Recreation
- Premier League