He threatened the 64-year-old man working alone at Ackroyd and Son Convenience store on Leeds Road after demanding cash.
Police said the robber attacked his victim with what is believed to be a crow bar before stealing cash and making his escape in the direction of Howden Clough.
The robbery happened at around 3pm on Monday July 4, but police only released details today.
The suspect is described as a stockily built 5ft 6in tall mixed race man. He was wearing a grey hooded top with the hood up, a black balaclava and black gloves.
Detective Constable Simon Thomas of Kirklees CID, said; “I would appeal to anyone who was around the area on Monday afternoon who may have seen anyone matching the suspect description, or who has any information to speak to police. Enquiries are ongoing.”
Anyone with information is asked to contact DC Simon Thomas at Kirklees CID via 101, quoting 13160285368.
Information can also be passed to Crimestoppers anonymously on 0800 555 111.