Police are appealing for witnesses to the attack on a man in his twenties who was at Offington Drive at the junction with Warren Road when the robbery took place on Tuesday at 4.15pm.
The victim was punched to the left eye causing cuts and swelling.
He fell off his bicycle and the suspect rode off on it towards the A27 westbound.
The bike is best described as a unisex adults mountain bike with a green/white frame and had a buckled rear wheel.
The suspect is described as a white male, between 18 and 22, 5’ 11” to 6’ 2”, of medium build and with short dark hair and dark eyes.
He was wearing a dark blue hooded sweatshirt, dark blue jeans, and white Nike trainers.
PC David Slade said: “This was a totally unprovoked attack on someone who was going about his normal business. It would have been busy at this time of day with passing vehicles. I am appealing for any witnesses to come forward in confidence.
“Anyone who witnessed the incident is asked to call 101 quoting serial 1019 of 12/11. Alternatively, call the independent charity Crimestoppers anonymously on 0800 555 111.”