The robbery happened at 7:09 p.m. Wednesday at L&H Grocery at 1346 Arch Street.
Investigators say the suspect held a black semi-automatic handgun with a silver-colored slide in his left hand.
He is described as a white male, about 30 years old, 5'11" tall with a medium build.
He was wearing a black jacket with a white logo in the upper left, a dark gray hoodie, black scarf and an ivory-colored knitted winter hat. He also wore light-colored blue jeans and tan work boots.
Police say the suspect may have fled in an older model gold or tan Nissan Maxima with tinted windows.
Anyone with information is asked to contact Norristown Police at 610-270-0977.