Barnsley skipper Shackell snapped up by rivals Derby
Rams boss Nigel Clough said: "Jason brings leadership and experience qualities to Derby County and it's a tremendous signing for us.
"We've had to beat off stiff competition to land his signature and we believe he will have a big part to play in our coming season and our future.
"He's in the prime of his career and was Barnsley's standout player last season, as well as being one of the most impressive defenders in the Championship. Having watched him a lot last season it's clear he's a player who lives to defend."