Three men went into the Betfred on Ramsden Street at about 9.40pm on Tuesday.
One demanded money from the tills, another stood near the counter and the third waited near the door of the premises.
The first man then jumped over the counter and went into the staff kitchen area looking through their personal belongings, taking a mobile phone.
All three men then fled the shop and went towards the town centre and down onto Queengate.
Police said the first suspect was described as black, in his early 20s, 5ft 10ins, of slim build and clean shaven. He had a deep voice.
The second suspect was described as black, in his early 20s, 5ft 11ins, of thin build and clean shaven. He was wearing a dark hooded top with white bands on the arms and the drawstrings pulled tight to his face.
The third suspect was described as 5ft 9ins and of thin build.
Anyone with any information or witnesses are asked to contact Kirklees CID via 101 quoting crime reference number 13180532229. Information can also be passed to Crimestoppers anonymously on 0800 555 111.