A Cinelli Hobootleg Geo mountain bike was stolen from the cycle racks in Fish Street in the town centre last week.
The bike, which is red with thick tyres, was secured to the rack and was taken sometime between 3.30pm and 4pm on Wednesday, 30 November.
Pictured is a bike similar to the one stolen, which has straight handlebars and a tan-coloured saddle.
Witnesses or anyone with information about the stolen bike is asked to contact Northamptonshire Police on 101. Alternatively, they can call independent charity Crimestoppers anonymously on 0800 555111.