Futsunushi (経津主神, Futsunushi-no-Kami, also 布都怒志命 or 布都努志命, Futsunushi-no-Mikoto), also known as Iwainushi (斎主神 or 伊波比主神, Iwainushi-no-Kami), is a warrior god in Japanese mythology.

[3] One theory interprets the futsu (Old Japanese: putu) in Futsunushi's name as an onomatopoeic sound of a sword swinging and cutting something.

[7] The name Iwainushi (historical orthography: いはひぬし, Ihahinushi; OJ: Ipapinusi) meanwhile is a contraction of iwai no ushi (斎之大人), 'master of worship'.

[3][8][9] A variant account of Izanagi and Izanami's begetting of various gods (kamiumi) cited in the Nihon Shoki states that when Izanagi killed the newborn fire god Kagutsuchi (whose birth caused the death of his wife Izanami), the drops of blood from his sword congealed to form the rocks by the heavenly river (天の安河, ame no yasukawa) from which Futsunushi was born.

The blood which dripped from the sword's hilt ring then turned into two gods named Mikahayahi-no-Kami (甕速日神) and Hihayahi-no-Kami (樋速日神); Mikahayahi is here identified as Takemikazuchi's parent.

The main narrative of the second volume of the Nihon Shoki relates that after the failure of the earlier messengers, Ame-no-Hohi and Ame-no-Wakahiko, to perform their mission, the gods of heaven headed by the primordial deity Takamimusubi decide to send Futsunushi, the son of Iwatsutsuno'o and Iwatsutsunome, as their new emissary.

[12] A variant account has Futsunushi and Takemikazuchi putting to death the evil deity Amatsumikaboshi (Kagaseo) in heaven first before they descend to Izumo.

The account adds that it was at this time that Iwainushi-no-Kami (possibly another name for Futsunushi), the deity enshrined in Katori, received the epithet iwai no ushi, 'master of worship.'

"[14]The Fudoki of Hitachi Province (modern Ibaraki Prefecture) also refers to a deity named 'Futsu-no-Ōkami' (普都大神) who is often identified with Futsunushi.

The story relates that the Sobataka deity, acting under the orders of the god of Katori, raided the land of Mutsu and stole 2,000 horses from the local kami.

When the god of Mutsu gave chase, the Sobataka deity drained Lake Kasumigaura using a 'tide-ebbing jewel' (干珠 kanju), allowing the horses to cross over to the other shore.

The Ryōjin Hishō compiled in 1179 (the late Heian period) attest to the worship of the gods of Katori and Kashima as martial deities at the time of its compilation: These gods of war live east of the barrier:[a] Kashima, Katori, Suwa no Miya, and Hira Myōjin; also Su in Awa, Otaka Myōjin in Tai no Kuchi, Yatsurugi in Atsuta, and Tado no Miya in Ise.

[29][30] A collection of medieval legends, the Shintōshū, identifies the Katori deity as a manifestation of the eleven-faced form of the bodhisattva Avalokiteśvara (Kannon).