The incident occurred about 1:30 AM at the National Credit Union, Regent and Oronoque Streets, Georgetown about 1:30 AM.
Police said that in addition to the cash, the robbers carted off five shotguns and three handguns.
An unarmed guard told investigators and reporters that he was tied up by three men, one of who was armed with a handgun and the other cutlasses. They then broke down a grill door, entered the building and accessed a safe that contained the cash.
The men escaped in a waiting car.
The guard said he managed to untie himself and called his chief security officer who summoned police to the scene.