Derby had also accepted an offer from City, but Bolder has chosen Gregory's side over a return back to Hull. He's signed a 2½-year contract at Loftus Road.
Phil Brown is now set to turn his attention towards signing a defender after failing in attempts to bring in midfielders David Prutton, Seth Johnson and Adam Bolder.
Brown has John Welsh returning to his squad after injury, but will again be without David Livermore for Tuesday's game against Leeds.