Four robbers, who wore balaclavas and scarves over their faces and dark clothing, entered through a rear door of McDonald's in Wildridings Road as staff were clearing up at 11.45pm on Tuesday.
They assaulted a member of staff, demanded money and stole a "substantial amount" of cash before leaving.
The restaurant in Wildridings Road was closed.
Detective constable Luke Simms, who is investigating, said: "I am keen to hear from anyone who may heave seen anyone acting suspiciously in the area at the time, or who may have information about the robbery."
Police are appealing for witnesses to contact them on 101 or call Crimestoppers anonymously on 0800 555 111.