The private bank was located within the limits of Saddar Police Station in Rawalpindi.
The guard was standing near the cash counter when he accidentally pulled a trigger and the bullet hit straight to the cashier Hina Gul. She died on the spot.
Another bank officer was also injured in the incident.
Police have arrested guard Umar Gul and registered a case. Police said they would investigate the case whether it was an accident or a crime.