The Royal Newfoundland Constabulary says someone with a knife robbed a Bank of Montreal branch on Elizabeth Avenue in St. John's just after noon on Monday.
A police spokesperson said investigators believe it was a man, but the person's face was masked and he or she wore a black-hooded jacket.
The thief demanded cash, ran off with an undisclosed amount of money, and was last seen running toward Freshwater Road.
No one was hurt in the incident.
Soon after the lunchtime robbery, officers with the criminal investigation, forensic identification, patrol services, and the police dog service of the RNC were at the site.
They visited surrounding businesses, took statements and looked for possible surveillance footage.
The bank was closed for the remainder of the day but the foyer was unlocked for ATM access by mid-afternoon.