He was not given a first-team chance at Blackburn, despite being there since he was eight.
Fielding, who had loan spells at Wycombe, Northampton, Rochdale and Derby, said: "I wasn't even getting on the bench so it was time to go.
"All I ever wanted was to play for Blackburn. Nothing against Derby, it's a great club and I'm delighted to be here, but I was going to watch Rovers as soon as I could walk.
"But their loss is someone else's gain, hopefully Derby's."