The robbery took place at 07:00 GMT at McColl's convenience store in Blandford Road, Poole.
Staff had a "terrifying ordeal" although no-one was injured, Dorset Police said.
Officers described the raiders as white, with one wearing a white bobble hat with a white scarf over his face.  The second wore a blue beanie hat.
They were carrying a camouflage army-style bag and a black bin liner and left through the back of the shop toward Manton Road.
Two men armed with knives threatened staff at a Dorset shop before stealing cash and cigarettes.