It happened at McColls in Whipton Village.
Police were called at around 5:30am on Friday 22 February.
It's believed the suspect was already inside the store when the female member of staff arrived to open up.
He made threats, assaulted the woman, then ran off with some money - the woman was not badly hurt.
Police have arrested a man his 50's on suspicion robbery.
Parking fees rise at two Devon attractions
Increase of 10 percent
Street hubs planned for Exeter
'They are a sleek and modern answer'
Sex abuse masseur jailed for four years
Ian Nicholls, 64, assaulted clients
'Safeguards-on-asylum-seekers' Reform council voted down
Even his seconder had second thoughts
Devon care home fined £200,000 after two residents drowned
Burdon Grange failed to provide safe care
Residents urged to help rough sleepers
Volunteers can join the soup run
