Shackell said on completing the deal: “I’m delighted to sign for Derby County and I’m really excited about the prospect of working with Paul Clement, his coaching staff and the rest of the players.
“There’s a really strong squad here and I’m happy to be part of it. Derby have been at the top end of the division for the last few years and I believe that we can be up there again this season, hopefully making that final step and reaching the Premier League.
“The club has made massive progress in recent times and after hearing the vision and ambition of Mel Morris, Sam Rush and Paul Clement, I couldn’t wait to sign.”
He added: “I know just how big a club Derby is following my previous spell here a few years ago and I’m well aware of the fantastic support that we have.”
The fee is undisclosed but Derby have used the £250,000 they recently won in the Sky Bet Transfer Fund. It is believed they have paid at least a further £3 million to take their former player back to Pride Park.