A robber armed with a silver handgun threatened to shoot a shop assistant in the Brighton Rock Shop by the West Pier.
The man demanded takings from the till, telling the 40-year-old woman: "Give me all your notes or I'll shoot you."
She handed over £50 and the robber ran off towards Queensbury Mews.
He is described as white, between 5ft 6in and 5ft 8in tall, in his early to mid-20s with dark brown hair.
He wore a shiny black jacket, blue jeans and white trainers with distinctive blue and white dots on the sides.
The robbery happened shortly before 1pm on Wednesday.
Detective Constable Graham Mepham of Brighton & Hove police said: "On a weekday lunchtime the coast road is always very busy. I urge motorists who think they saw a man fitting the description in the area to call me."
Witnesses should call 0845 6070999 or Crimestoppers in confidence on 0800 555111.
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