The thief, who was said to be carrying a knife, left the One Stop shop in Middle Park Way, Havant, with more than £500.
A police spokesman said: '˜Between 8.35pm and 8.40pm last night (18 October), a man who was said to be carrying a knife made threats towards a member of staff while demanding money.
'˜The man left the shop carrying a till casette which contained more than £500.
'˜Enquiries are on-going.'
Anyone who witnessed the incident is asked to contact police on 101, quoting 44180392847.