A kunai (苦無 kunai) is a Japanese tool possibly derived from the masonry trowel. Two variations are the short kunai (小苦無 shō-kunai) and the big kunai (大苦無 dai-kunai). It is a good example of a basic tool which, in the hands of a martial artsexpert, could be used as a multi-functional weapon. It is commonly associated with the ninja, who used it to gouge holes in walls.The kunai was conventionally wrought in ranges from 20 cm to 60 cm, with the average at 40 cm. The kunai was used by peasants as multi-purpose gardening tools and by workers of stone and masonry. The kunai is not a knife, but something more akin to a crowbar. The blade was soft iron and unsharpened because the edges were used to smash plaster and wood, to dig holes and to pry. Normally only the tip would have been sharpened. The uses a kunai was put to would have destroyed any heat-treated and sharpened tool like a knife.

Kunai normally had a leaf-shaped blade and a handle with a ring on the pommel for attaching a rope. This would allow the kunai's handle to be wrapped to act as a grip or, when used as a weapon, to be strapped to a stick as an expedient spear; to be tied to the body for concealment; or to use as an anchor or piton. Contrary to popular belief, they were not designed to be used primarily as throwing weapons, though they could be thrown to cause damage. Instead, they are a thrusting and stabbing implement.