The assaults took place after three men entered the BP Euro Garages site, on the A50 near Uttoxeter, at 12.45am.
One of the men jumped over the counter and threatened the assistant before striking him. A lorry driver who was inside the building at the time was punched in the face by a second robber.
The three robbers, two of whom had bolt croppers, stole cigarettes, cash and scratch cards before escaping in a car police believe was a black Vauxhall Vectra.
Neither the shop assistant nor the lorry driver required hospital treatment.
A Staffordshire Police spokeswoman asked anyone with any information to contact police. She said: “The men are described as white, aged in their early 20s and wearing hooded tops. One was wearing a pair of shorts.
“An investigation into the incident is now underway and anyone with information is asked to contact Staffordshire Police on 101, quoting incident number 24 of March 13, or Crimestoppers anonymously on 0800 555111.”