Ōyama-tsumi (Old Japanese: Opoyama-tumi; Kojiki: 大山津見神 or Nihon Shoki: 大山祇神, 大山積神, 大山罪神), also Ōyama-tsumi-mi'oya-no-mikoto (大山祇御祖命), is a god of mountains, sea, and war in Japanese mythology. He is an elder brother of Amaterasu, Tsukuyomi, and Susanoo. His other names are Watashi-no-Ōkami (和多志大神) and Sakatoke (酒解神).

In the Kamiumi section of the Kojiki, Ōyama-tsumi was born between Izanagi and Izanami. With the female deity Kaya-no-hime (鹿屋野比売神), also known as No-zuchi (野椎神), he fathered eight kami (deities):

He also rendered several offspring without a mother:

In later genealogical traditions, particularly some medieval charts that represent variant passages of the Nihon Shoki, but importantly not in the Kojiki, nor the main Nihon Shoki narrative themselves, Ashinazuchi (or Ashinazuchi (Kojiki: 足名椎; Nihon Shoki: 脚摩乳命; 足名鉄神), also known as 足摩乳命 or 足名槌命 (Ashinazuchi-no-mikoto or Ashinadzuchi-no-mikoto)) and Tenazuchi (or Tenazuchi; Kojiki: 手名椎; Nihon Shoki: 手摩乳命), also known as 名槌命 (Nazuchi-no-mikoto) were siblings fathered by Ōyamatsumi. In the Kojiki, Ashinazuchi and Tenazuchi are simply introduced as an elderly married couple living in Izumo with their last surviving daughter (Kushinadahime). In the Kojiki, no parentage is given for them. This discrepancy stems from the Kojiki and Nihon Shoki being primarily political documents and not having been fully harmonized. Furthermore, medieval schools such as Ise, Yoshida, and Suika, and later Kokugaku scholars, reworked genealogies extensively.

However, in the Nihon Shoki, Ōyama-tsumi is supposed to be born when Izanagi slashed his child, Kagutsuchi (軻遇突智).

The child of Ōyama-tsumi from his first wife Kaya-no-hime, the deity Ame-no-sagiri has a daughter, Tohotsumachi-ne (遠津待根神), and the eighth descendant of the male deity Ōkuninushi (大国主神), the male deity Ame-no-hibara-ōshinadomi (天日腹大科度美神), from their union gave birth to the male deity, To'otsuyama-sakitarashi (遠津山岬多良斯神), who is the descendant of Ōyama-tsumi.

There is not much written about Ōyama-tsumi, and children associated with him appears at times. As for the myth of Yamata-no-orochi, Susanoo's wife, Kushinada-hime (櫛名田比売命 Kushinada-hime-no-mikoto), and her twin parents, the male deity Ashinazuchi (足名椎) and female deity Tenazuchi (手名椎), are known and claimed to be the children of Ōyama-tsumi.

Afterwards, the lineage falls together with his descendants of his half-brother Susanoo, with the union of Ōyama-tsumi first daughter, Kamu'ō-ichi-hime (神大市比売), between them gave birth to Ōtoshi (or Toshigami) and Uka-no-mitama. Then, Susanoo's union with Ōyama-tsumi granddaughter, Kushinada-hime, gave birth to the male deity Yashimajinumi (八島士奴美神). Then Yashimajinumi married Ōyama-tsumi's daughter Konohachiru-hime (木花知流比売), from their union gave birth to the male deity Fuwanomojikunusunu/ Fuwanomodjikunusunu (布波能母遅久奴須奴神, or Fuhanomojikunusunu/ Fuhanomodjikunusunu). Fuwanomojikunusu is the husband of Hikawa-hime (日河比売, or Hikaha-hime), the daughter of the male deity Okami (淤加美神), from their union gave birth to the male deity Fukafuchi-no-Mizuyarehana (深淵之水夜礼花神).