The Sai is a three-tined metal fork with a weighted handle. It is made of metal and has a round or octagonal section. The origin of the Sai is not known, though similar weapons were found in southern China and Indonesia.

The Sai is used in pairs and generally follows the arm movements. It is primarily used as a defensive weapon. When aligned along the forearm, it can ward off blows. As an offensive weapon, it can be used for jabbing with either end, used as a truncheon by swinging it around and grasping the central tine striking as with a club, or using the tines to hook the vital areas of an opponent. Although not weighted for it, the Sai can be thrown, and one training form teaches how to do this.

Sais were issued to elite units of the Okinawan police prior to 1870. Each policeman carried three, in case one was lost during a struggle. As a farm tool, the Sai was used to secure the tongue to a cart or wagon as well as a pitchfork.