The 37-year-old man, who was attacked from behind, was taken to hospital after being hit as he was locking up Yorkshire Main Miners' Welfare Club in Edlington.
A large quantity of cash was stolen in the raid, which took place in the early hours of yesterday morning.
Police are now appealing for witnesses to the incident which left the man in hospital with minor head injuries.
The attack took place just after midnight on Monday at the club in Edlington Lane.
The member of staff was closing down the premises when he was approached and hit over the back of the head, knocking him unconscious and causing him to fall to the floor.
Whilst he was unconscious, entry was forced into the club and a large amount of money was stolen.
Due to being hit over the back of the head, the employee did not see how many offenders were involved and cannot provide a description.
Detective Constable Fiona Bowling who is leading the investigation said: “An investigation is currently ongoing into the incident and we have been conducting enquiries in the area.
"Due to the lack of description, we are now appealing for anyone who may have witnessed the incident, or saw any suspicious activity in the area, to please report it to us.
“Thankfully the victim only suffered minor injuries in the incident, however his injuries could have been much worse."
Anyone with any information, even if they think it might be irrelevant is urged to report it via either 101 quoting incident number 14 of 2 October, or anonymously through Crimestoppers on 0800 555 111.