A 50-year-old man handed over a "substantial" sum of cash when he was confronted by three masked knifemen at his office door.
The gang threatened the man when he opened the door of the business in London Road, Ashford, Surrey, at about 2045 GMT on Friday.
After telling him to hand over any items of value, the raiders ran off in the direction of Ashford Hospital.
"This was a very serious incident," said Det Con Simon Egan.
Dog unit search
"It must have been terrifying for the victim. Fortunately he was not injured, although he has obviously been left shaken by the robbery."
Officers and a police dog unit carried out a search of the area but did not find the knifemen.
House-to-house inquiries were also carried out in the area.
Det Con Egan appealed to anyone who saw or heard anything out of the ordinary to contact police.
The suspects are all described as being white and between 5ft 7in and 5ft 8in tall.