The robbery happened shortly after 3pm at the Dunfermline Building Society in Gilmour Street, Paisley, Renfrewshire, yesterday.
Police said a 25-year-old female employee was not hurt but "extremely traumatised" after being threatened with a blade.
Detectives believe the same man may have struck at the Dunfermline Building Society in nearby Johnstone last month.
He is described as white, aged 45-50, and thin with grey hair and stubble on face. He was wearing a black waterproof jacket.
Detective Inspector David Wagstaff from Police Scotland said: "I'd like to speak to anyone who was in the Gilmour Street area around 3pm on Friday and who may have seen this man hanging about prior to the robbery taking place or running off afterwards.
"The description of the suspect is very similar to another robbery we are investigating which occurred at the Dunfermline Building Society in Johnstone on November 25. The suspect in both robberies was described as being very nervous.
"I like to ask people to think back to Monday November 25 as well as yesterday to see if they know of anyone who matches the description and who may suddenly have acquired a sum of money after being been a bit short in the days before."
Anyone with information is asked to contact Police Scotland on 101.