The 28-year-old will return to the capital where he spent the 2006 and 2007 seasons with Harlequins before making the switch to Humberside.
Mills' arrival comes as props Danny Ward and Ben Jones picked up injuries in last Sunday's Carnegie Challenge Cup tie against St Helens. Both are expected to be out for 2-3 weeks after Ward damaged cartilage in his leg and Jones damaged bone in his arm.
Harlequins head coach, Brian McDermott, said today:
"David has been with us in the past and it was a difficult decision to let him go. I remember saying to him that we may work again together one day and as fortune would have it, here we are again.
"He has been on periphery at Hull KR of late so now he has an opportunity to impress us and his employers by showing what he can do at Super League level."
Mills will be available for this Sunday's engage Super League fixture against Castleford Tigers which kicks off at 2pm at the Twickenham Stoop.
Tickets are available now from just £15 adults, £10 concessions, £5 under 16s by calling 0871 527 1315 and online at HERE.
