The incident occurred at a U.S. Bank branch located at NE 87th and 35th Ave. NE at around 6 p.m. Wednesday.
A man wearing a ski mask walked into the bank, pointed his gun at the woman and her son and demanded that the woman hand over her wallet.
He then walked up to a bank teller and demanded cash before fleeing the scene.
The thief is described as approximately 6-foot-1 and 170 pounds. He is also believed to be responsible for recent robberies of banks in Seattle and Bellevue.
Anyone with information on the man's identity or whereabouts is urged to call police immediately at 206-684-5540.