A man wearing a hoody and armed with a knife made off with an undisclosed amount of cash from a shop in Mosta, the police said.
The hold-up took place at 5.30pm in Saint Anne Street.
The police said the man demanded cash from the salesgirl, an 18-year-old, and made off with the money.
Magistrate Natasha Galea Sciberras is leading an inquiry.