The 25-year-old right-back joins the Bluebirds in a deal believed to be around £1.5m and will link up with the squad ahead of Saturday's pre-season friendly at Cheltenham Town.
Brayford enjoyed a successful spell at Pride Park, making 117 appearances for County, and was a regular in the Rams' defence last season.
But he admits the move to Cardiff was too good to turn down and is relishing the challenge of playing in the Premier League.
Brayford told the club's official website: "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."