The Can-am Renegade 800 XXC was stolen between 1.50pm and 3.30pm on Saturday 30 June 2012, from a lay-by near Fewston reservoir.
The quadbike, which has the registration number YN61HAU, is black with a custom set of Makita decals, a triangular front bumper and an aluminium underside.
PC Janine Walker, of Harrogate Safer Neighbourhood Team, said:
I would like to speak to anyone who can help to recover the quadbike and ask that they contact the police or Crimestoppers as soon as possible.
Anyone who can help the police with their enquiries is asked to contact Harrogate police on 101, press option 2 and ask for Janine Walker or by email at firstname.lastname@example.org.
Or call Crimestoppers anonymously on 0800 555 111 quoting reference number 12120105508.