The robber was last seen running west on Sixth Street, behind the store, 110 E. 6th St.
There were customers in the store at the time of the robbery, reported about 3:47 p.m., but no one was injured, the lieutenant said.
The invader was dressed in a gray, hooded sweatshirt he had pulled over his ball cap, denim jeans and tennis shoes, Pacifico said. The robber looked to be 30 to 40 years old, 5 feet 10 to about 6 feet tall and roughly 250 pounds.
Pacifico said the sweatshirt had a large design on the back and one on the left front chest. Both appeared to be embroidered.
If you saw the man or have any information to offer police, you are asked to call Franklin police Detective Jeff Stewart at 937-746-2882.