Ninja (??) were said to be agents of espionage and assassination in feudal Japan in legend and popular fiction. Some modern practitioners of budo ninjutsu argue that ninja were used primarily as spies, not assassins; and this appears to be borne out by some historical records. It is popularly believed that the ancient ninja were peasants, forbidden under law from studying the samurai swordplay techniques because of feudal Japan's caste structure; but more likely they originated from a variety of castes.

Ninja are said to have made use of weapons that could be easily concealed or disguised as common tools. Weapons commonly attributed to them included shuriken and bo. In popular folklore, ninja also used special short swords called ninja-ken (or ninja-to see below for explanation). Ninja-ken are smaller than katana but larger than wakizashi. The ninja-to was more of a utilitarian tool than a weapon. Another version of the ninja sword was the shikoro ken (saw sword). The shikoro ken was said to be used to gain entry into fortresses. The shikoro ken supposedly could also be used to cut (or saw) through opponents.