Niten Ichi-ryū (二天一流), which can be loosely translated as "the school of the strategy of two heavens as one", is a koryū (ancient school), transmitting a style of classical Japanese swordsmanship conceived by the warrior Miyamoto Musashi. Hyōhō Niten Ichi-ryū is mainly known for the two-sword—katana and wakizashi—kenjutsu techniques Musashi called Niten Ichi (二天一, "two heavens as one") or Nitō Ichi (二刀一, "two swords as one").

Musashi originally studied Enmey Ryū and Tōry Ryū, which were ryūha founded by his grandfather Miyamoto Musashi no Kami Yoshimoto and his father Miyamoto Muninosuke respectively. Musashi eventually focused in the kenjutsu and nitōken and developed his own style.

Around 1640, Musashi intended to pass on his art to three successors from among his thousand students; specifically, to Terao Magonojo, his younger brother Kyumanosuke and to Furuhashi Sozaemon. He considered Magonojo to excel in technique but to lack in reflection, while Furuhashi excelled at reflection but lacked technique. Magonojo received the treatise, the Go Rin no Sho. Hosokawa Mitsuhisa made two copies, one for Furuhashi and one for himself, which he transmitted under the name of Ihon go rin no sho. The best known edition today is this Hosokawa copy.

Magonojo then yielded the role of successor to his younger brother Kyumanosuke who had received the Hyoho San-jugo from Musashi. It was Kyumanosuke who transmitted this document to his students with seven added instructions called the Hyoho shiji ni kajo.

Shortly before his death, Musashi also wrote the Dokkodo ("Going My Way"). It seems to be a list of rules that one should try to follow in life steeped in Buddhist precepts.

Hyoho Niten Ichi-ryu
This is an old video of Niten Ichi ryu demonstrated by the Imai Masayuki, the late soke. Imai demonstrates the tachi, kodachi & nito techniques of Niten Ichi Ryu.