Memphis police said the thief initially appeared to be spreading Christmas cheer.
"At approximately 10:00am, a lone male wearing a Santa mask entered the Memphis City Employee Credit Union," police said in a statement.
"Once inside, the male handed candy canes to several employees and customers and wished them a merry Christmas."
The man then approached the teller and handed her a note demanding money, police said.
After obtaining the cash, the thief fled.
Police said no-one had been arrested over the incident and the investigation was ongoing.