The man went into the Westmont Ameriserv Bank branch on Minno Drive just before 6pm Thursday.
The white male showed a handgun, demanded money and police say even jumped on the counter when the teller didn't respond quickly enough.
Witnesses say he had a makeshift mask and was wearing dark sunglasses, an orange hat, khaki pants and a grey t-shirt.
They also noticed a Maltese Cross tattoo on his left calf as he ran out the back door with an undetermined amount of cash.
Anyone with information is asked to call West Hills Police at 814-255-4145.