A silver mountain bike was stolen from the cycle rack near the fountain in Market Square, Northampton.
The victim left the cycle locked between 1.30pm and 3.10pm on Wednesday, December 2, but when he returned, both the lock and the cycle were missing.
The make of the bike is orange and it has front suspension and disc brakes.
Anyone with any information can call Northamptonshire Police on 101. Alternatively, they can call the independent charity Crimestoppers anonymously on 0800 555 111.