The holdup at the E-Z Mart on West Markham happened shortly after eight o'clock.
When officers arrived at 2711 West Markham, the cashier told them one man pointed a handgun at his face and demanded money while the other held the door open and served as a lookout.
After getting the cash, the men then ran west from the store down Markham Street.
The suspects are described as being black men, one of them about 5'6" tall, the other about 5'9" tall, and both weighing about 160 pounds.
They were both wearing all black clothing and covered their faces from the nose down with black bandanas, police said.