Police spokeswoman Kim Kobel said the suspect entered the Bustop and held a gun to the manager's head. The manager was able to chase the man off, and he left running northwest.
Police believe the manager and possibly the owner were the only people in the building at the time.
Kobel said the suspect is described as a white man between 20 and 30 years old, around 5-feet, 10-inches tall and around 180 pounds. He may have been heavy-set or he may have been wearing something bulky under his clothing. He was wearing black pants, a black hoodie and dark sunglasses. His face was obscured, possibly with a ski mask.
Police searched the area for the suspect, but now believe that he may have fled Boulder.
Anyone who thinks they may have seen the suspect is asked to call police, Kobel said.