A kunai (苦無, kunai) is a Japanese multipurpose tool and weapon thought to be originally derived from the masonry trowel.

A kunai normally had a leaf-shaped wrought blade in lengths ranging from 20 and 30 cm (7.9 and 11.8 in) and a handle with a ring on the pommel for attaching a rope.

The attached rope allowed the kunai's handle to be wrapped to function as a grip, or to be strapped to a stick as a makeshift spear; to be tied to the body for concealment; to be used as an anchor or piton, and sometimes to be used as the Chinese rope dart.

Varieties of kunai include short, long, narrow-bladed, saw-toothed, and wide-bladed.

The two widely recognized kinds are the short kunai (小苦無 shō-kunai) and the big kunai (大苦無 dai-kunai).

In some cases, the kunai and the Nishikori, a wide-bladed saw with a dagger-type handle, are difficult to distinguish.

Contrary to popular belief, kunai were not designed to be used primarily as throwing weapons. Instead, kunai were primarily tools used as weapons for stabbing and thrusting .

The kunai was originally used by peasants as a multi-purpose gardening tool and by workers of stone and masonry.