Ninjutsu (忍術^?) sometimes used interchangeably with the term ninpō (忍法^?) is the martial art, strategy, and tactics of unconventional warfare and guerrilla warfare as well as the art of espionage purportedly practiced by the shinobi (commonly known outside of Japan as ninja).

While there are several styles of "modern ninjutsu," the historical lineage of these styles is disputed. Some schools and masters claim to be "the only true and legitimate heirs" of the art, but ninjutsu is not totally centralized like modernized martial arts such as judo or taekwondo. Togakure-ryū is said to be the oldest recorded form of ninjutsu dating to the 1500s.

The main character nin (忍^?) is a phono-semantic compound composed of two greater characters. The upper character ha or jin (刃^?) is the phonetic indicator; its meaning of "edge of the sword" is therefore irrelevant here. The lower character kokoro or shin (心^?) means "heart" or "soul". The compound means "stealth", "secrecy", "endurance", "perseverance", and "patience".Jutsu (術^?) means "art" or "technique". Hō (法^?) meaning "knowledge", "principle", "law" or "system" when found with the prefix "nin" carries the meaning of ninja arts, higher order of ninjutsu.