The guard was carrying a cash box into the foyer at NatWest Bank, on the Bull Ring, when he was approached from behind by an unknown man who hit him over the head shortly before 10.20am.
Police say the guard, who was wearing a helmet, was knocked to the floor but was uninjured.
The male offender then grabbed hold of the cash box before fleeing the scene and getting into a Blue Nissan Qashqai, driven by another male, which was waiting nearby.
However, while he was running to the vehicle the suspect dropped the cash box and contents which have been recovered.
The vehicle then drove away from the scene, initially travelling through the Market car park and onto Chester Way. It was later found abandoned in Weaver Road, Castle.
Police describe the suspect as a white male, of slim build, who was wearing a grey hoodie, dark trousers and a black puffer jacket without sleeves. He was also wearing a black baseball cap and his face was covered.
The driver of the vehicle is described as a black man, who was wearing a red jacket, possibly checkered, and dark trousers.
Investigating officer Detective Sergeant Lianne Macfarlane said: “This was a serious incident and we are committed to doing all that we can to trace the people responsible.
“The investigation in its early stages and we are following a number of lines of enquiry. As part of the investigation I’m keen to speak to anyone was in Northwich town centre around the time this incident occurred and believes that they may have witnessed anything suspicious.
“The Bull Ring is a busy junction within Northwich and I believe that there will have been lots of people in the area who may hold vital information. I’m also keen to hear from anyone who believes that they may have seen the suspects after they abandoned the vehicle in Weaver Road, Castle.”
Anyone with any information in relation to the incident is asked to contact Northwich Police on 101 quoting incident number 273 of 29/8/17. Information can also be reported anonymously to Crimestoppers on 0800 555 111.