The 23-year-old winger, who had a spell on loan with the Rams last season, has agreed a four-year contract after the clubs agreed an undisclosed fee.
County head coach Paul Clement told the club website: "I'm delighted that we have been able to sign Tom on a permanent basis
"I'm aware of the positive impact he had on the side during his time on loan last season from speaking to people at the club, and also watching a number of the matches back myself.
"He is an exciting talent, there's no doubt about it, and he's another very positive addition to the squad."
Ince is the sixth player to join Derby this summer following the arrivals of Darren Bent, Alex Pearce, Scott Carson, Chris Baird and Andreas Weimann.
Source : PA