The victim was playing with friends in the grounds of the Hydro Leisure Centre on Jennyfields Drive at around 7pm on Friday 17 August 2012 when his black, white and purple X-Rated BMX was taken after he had left it unattended.
The bike, which is valued at around £300, has a black seat with a white X logo on it.
Officers are appealing for anyone who witnessed the theft or has any information about the whereabouts of the bike, to come forward.
PC Emma Newby, of Harrogate police, said:
I am urging anyone who was in the area at the time of the theft and saw anything suspicious to get in touch with the police or Crimestoppers.
I would also like to hear from anyone who has seen the bike around since it was stolen or anyone who has been offered it for sale.
Anyone who can help officers with their enquiries is urged to contact North Yorkshire Police on 101 – select option 2 – and ask for Emma Newby or Harrogate police.
Alternatively, Crimestoppers can be contacted anonymously on 0800 555 111.
Please quote reference number 12120136526 when passing information about this incident.