Ince spent a successful loan spell at the iPro Stadium in the second half of last season and the move has now been made permanent.
The 23-year-old scored 11 times in 18 appearances for the Rams, with his performances seeing him named the Sky Bet Championship Player of the Month for April.
Ince becomes the sixth player to join Derby this summer, following the arrivals of Darren Bent, Alex Pearce, Scott Carson, Chris Baird and Andreas Weimann in June as new manager Paul Clement shapes a squad to put the disappointment of the last two seasons behind them.
“I’m delighted that we have been able sign Tom Ince on a permanent basis,” Derby head coach Paul Clement told the club’s official website.
“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.
“I feel that we’re in a good place with how the squad is looking right now and credit to Sam Rush and Chris Evans for the work they have concluded, bringing in six quality signings this summer.”