, also pronounced Wadatsumi, is a legendary kami (神, god; deity; spirit), Japanese dragon and tutelary water deity in Japanese mythology. Ōwatatsumi no kami is believed to be another name for the sea deity Ryūjin (龍神, Dragon God) and also for the Watatsumi Sanjin, which rule the upper, middle and lower seas respectively and were created when Izanagi was washing himself of the dragons blood when he returned from Yomi, "the underworld".
The earliest written sources of Old Japanese transcribe the name of the sea god in a diverse manner. The c. 712 CE Kojiki (tr. Basil Hall Chamberlain 1883) writes it semantically as 海神 lit. "sea god" and transcribes it phonetically with man'yōgana as Wata-tsu-mi, 綿津見, lit. "cotton port see" in identifying Ōwatsumi kami and the Watatsumi Sanjin. The c. 720 CE Nihongi (tr. William George Aston 1896) also writes Watatsumi as 海神 "sea god", along with 海童 "sea child" and 少童命 "small child lords" for the Watatsumi Sanjin. In the modern Japanese writing system, the name Watatsumi is usually written either in katakana as ワタツミ or in kanji phonetically 綿津見 or semantically 海神 "sea god".
Note that in addition to reading 海神 as watatsumi, wata no kami or unagami in native Japanese kun'yomi pronunciation, it is also read kaijin or kaishin in Sino-Japanese on'yomi (from Chinese haishen, 海神, "sea god"). The original Watatsumi meaning "tutelary deity of the sea" is semantically extended as a synecdoche or metaphor meaning "the sea; the ocean; the main".
The etymology of the sea god Watatsumi is uncertain. Marinus Willem de Visser (1913:137) notes consensus that wata is an Old Japanese word for "sea; ocean" and tsu is a possessive particle, but disagreement whether mi means "snake" or "lord; god". "It is not impossible" he concludes, "that the old Japanese sea-gods were snakes or dragons." Compare the Japanese rain god Kuraokami that was similarly described as a giant snake or a dragon. The comparative linguist Paul K. Benedict proposed (1990:236–7) that Japanese wata, 海, "sea" derives from Proto-Austronesian *wacal, "sea; open sea".