It was taken from the Castel area, close to Saumarez Park, sometime between 22:00 on Thursday 13 August and 07:00 the following morning.
Police said the bike had a white drinks holder attached to the frame.
Anyone with information or who witnessed the theft is asked to contact PC 160 Carre on 725111 or alternatively call Crimestoppers anonymously on 0800 555 111.