The sai (Japanese: 釵, lit. 'hairpin'; Chinese: 鐵尺, lit. 'iron ruler') is a melee weapon used for stabbing, striking and disarming opponents. It came to international attention through Okinawan kobudō, elements of which spread to Japan, then to the wider world, when Karate became popular in the mid 20th Century. The basic form of the weapon is that of a pointed metal prong with two shorter metal side prongs (yoku) projecting from the handle (tsuka).

Before its adoption in Okinawa, similar weapons were already being used in other Asian countries including India, Thailand, China, Vietnam, Malaysia, and Indonesia.^[1] The basic concept of the sai may have been brought to Okinawa from one or several of these places over time.^[2] Some sources theorise that the weapon may be based on the Indian trisula, an ancient Hindu-Buddhist symbol^[3] that may have spread along with Hinduism and Buddhism into South-East Asia.^[4] The word trisula itself can refer to either a long or short-handled trident.

In Okinawa the sai was used by domestic police (ufuchiku) to arrest criminals and for crowd control. Use of the sai in Okinawan martial arts was improved in 1668 by Moto Chohei, an Okinawan prince.^[5]

Japan had a similar weapon, the jitte, which was originally used as a non-bladed weapon by guards in the Shogun's palace, and was subsequently issued to senior officials as a badge of office. Edo period examples of the jitte typically have only a single prong. The relationship between the sai and jitte is unclear.

• Monouchi, the main prong of the sai, this can be round or faceted.
• Yoku, the prong like side guards which are usually symmetrical, with the exception of the manji design sai developed by Taira Shinken which employs oppositely-facing yoku resembling the swastika (manji).
• Tsume, the sharp tip of the side prong (yoku).
• Moto, the actual center point between the two side guards.
• Tsuka, the handle of the sai. The tsuka can be wrapped with different materials such as cord or ray skin (same) to provide a grip. This tsuka is 5 inches (13 cm) long.
• Tsukagashira, the butt end of the handle (tsuka).
• Saki, the sharp tip of the sai.^[6]

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Performances of martial arts forms using sai typically use the weapon in pairs, with one in each hand.^[1] In modern Okinawan Kobudo, five kata (choreographed patterns of movements in martial arts) are commonly taught, including two kihon kata.

The utility of the sai as a weapon is reflected in its distinctive shape. It is a weapon used for fast jabs and strikes but it also has many defensive techniques. These techniques include a variety of blocks, parries and captures against attackers from all directions and height levels. Use of the sharp tip, main prong and pommel is emphasized, as well as rapid grip changes for multiple jabs and strikes.

One technique commonly depicted is the use of one of a sai's hooks to entrap and lock an attacker's blade. Some variations of sai have their hooks pointing inwards towards the main prong to facilitate this maneuver. While this does not completely immobilize the attacker, it encumbers them in close quarters.

Because there is no morphological plural in Japanese, the word "sai" refers to either a single weapon or multiple. Nicho sai refers to a kata that uses two sai, while sancho sai kata refers to kata using three sai.

• Nunti Bō
• Okinawan kobudō
• Parrying dagger
• Trident