The 21-year-old Tottenham Hotspur player shone on loan at the Gers last season.
Dominic Ball in action with Kilmarnock's Craig Slater (L) and Greg Kiltie
Ball was at Ibrox last season to help the Gers complete their long-awaited return to the Scottish Premiership, playing an important role both at the back and in defensive midfield.
Rangers boss Mark Warburton was delighted with the 21-year-old's contribution and was reportedly pursuing him this summer, either on another loan or on a permanent deal according to the Yorkshire Post, only for the Millers to jump in and get the deal done and dusted.
Dominic Ball at Tottenham Hotspur
The player joins Rotherham on a three-year deal, with former Celtic favourite Alan Stubbs managing to talk Ball into heading to the Championship rather than the Scottish top flight.
The England Under-20 man was given a couple of chances to impress for Spurs on their recent pre-season trip to Australia, but Ball struggled in central defence against Juventus before looking much more comfortable anchoring midfield in a short outing against Atletico Madrid.
Tottenham's Ryan Mason leads Dominic Ball, Michel Vorm and co onto the pitch against Juventus
But it was enough for Mauricio Pochettino decide the youngster's career would be best continued elsewhere, and Millers fans will be delighted that they have managed to secure another Spurs starlet, having missed out on a permanent deal for Grant Ward earlier in the summer.