A man wearing a gas mark threatening a Co-op staff member with a knife and stole cash from the till.
Police said the man walked into the store in San Diego Road, Gosport, between 9am and 9.20am on Tuesday and demanded money.
Fearing for their safety the staff member handed over some cash and the man left the store.
No-one was injured.
The man was described as slim to average build, about 6ft 2ins tall, wearing a gas mask, and black clothing with a hood up.
Police want to speak to anyone who saw what happened, or saw the man in the area at the time.
Detective Constable Julia Nicol said: ‘This would have no doubt been a terrifying incident which left the victim very shaken.
‘We would like to hear from witnesses as soon as possible who may have seen this man in the area.
‘We know he was playing with a bike outside the shop prior to the robbery. He was wearing very distinctive clothing, namely the war-type gas mask, so you may have noticed this.
‘If you saw anything, please call us.’
Anyone with information is asked to contact DC Nicol on 101, quoting 44180039139, or call the charity Crimestoppers anonymously on 0800 555 111.