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 has long been a target for Alan Pardew, but the manager was not allowed to sign him as Mike Ashley prefered to bring in foreign players from the "bargain bin".
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.