According to Sussex Police, the pensioner was approached by a woman in Fitzalan Road on Friday (February 12) at around 2.30pm.
She snatched his Rolex watch from his wrist, police said, causing a minor injury.
The woman, who is described as slim, 5ft 5ins tall and wearing a colourful dress, then made off towards East Street.
Detective Inspector Kimberley May said: “This was extremely distressing for the victim and we are appealing for any witnesses to come forward or if anyone is offered a Rolex watch for sale to contact us.
“We have increased our patrols in the area and if anyone has concerns please speak to one of our officers.”
Anyone who witnessed the incident is asked to report online or ring 101 quoting serial 694 of 12/02.