A man in his 80s saw a man ride off on his bicycle when he was shopping.
Police are issuing a CCTV photo of the man, wearing an "unusual straw hat and hope that someone will be able to name him".
The stolen bike was a Marin with a silver frame and mudguards, with a silver carrier on the back, front suspension, back and front lights, a mileage computer on the handlebars and the saddle is split on both sides.
The suspect is described as wearing a dark T-shirt, light coloured long shorts, dark trainers and a straw trilby type hat. He rode off in the direction of Plume Avenue.
This happened around 3.40pm near Boots in West Parade, Plume Avenue, Colchester, on August 7.
Anyone who recognises the man who knows the current location of the bike is asked to contact Pc Melanie Wilson at Colchester Police Station on 101.