The woman and two other female colleagues also tried to fight the men off as they robbed the Co-op in Whitehall Road, Drighlington.
Detective Inspector Richard Holmes, of Leeds District CID, said: “This was a truly terrifying experience for the victims, particularly the woman who was viciously attacked and seriously injured.
“She and her colleagues have been left understandably very distressed by this incident and we urgently need to identify the men responsible."
The robbers wore balaclavas when they entered the shop shortly after 10.05pm and threatened the three female members of staff.
The men went on to demand access to the safe but the victims refused and tried to fight the men off.
One of the women was repeatedly hit in the face and suffered a fractured cheekbone and significant bruising and swelling which required hospital treatment.
Another of the woman was hit on the head by the same suspect but was not injured.
While the women were under attack, one of the other suspects forced open the tills and removed cash.
The three men then left the scene in a silver Nissan Juke, which was driven by a fourth male.
The car was displaying the false registration DN57 MXJ, which links to a different vehicle.
Det Insp Holmes said: “We are carrying out extensive enquiries and we would like to hear from anyone who witnessed the incident or who saw the men or the vehicle in the area at any time leading up to or after the robbery.
“That vehicle is still outstanding and we would like to hear from anyone who has seen it parked up or abandoned anywhere.”
Anyone with information is asked to contact Leeds District CID via 101, quoting crime reference 13180427412.
Information can also be passed on to the independent charity Crimestoppers anonymously on 0800 555 111.
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