Ryujin (龍神) is the god of the sea in Japanese mythology. He is a dragon, a symbol of the power of the ocean, with a wide mouth and the ability to take human form. He lives in Ryūgū-jō from where he controls the tides with magical gems. Sea turtles, fish, and jellyfish are often described as his servants.
Ryujin is the father of the beautiful goddess Otohime who married the hunter prince Hoori. The first emperor of Japan, Jimmu, is said to be the grandson of Otohime and Hoori, so Ryujin is considered one of the ancestors of the Japanese imperial dynasty.
According to legend, Empress Jingū carried out her attack on Korea using Ryujin's jewels. During a confrontation with the Korean navy, Jingū threw the low tide jewels into the sea and the tide receded, grounding the enemy ships.
When the Koreans got out of their boats, Jingū threw the high tide jewels into the sea: the sea rose and drowned the Korean soldiers. An annual festival, Gion Matsuri, is held at Yasaka Shrine to celebrate this legend.
Another legend tells how the jellyfish lost its bones. According to this story, Ryujin wanted to eat the liver of a monkey (in some versions of the story, in order to cure an incurable rash, in others, at the request of his wife) and sent the jellyfish to bring him one.
When the jellyfish found a monkey and asked him for his liver, the monkey claimed that it was in a vial hidden in the forest: it was too heavy and the monkey preferred to hide it during the day.
He proposed to the jellyfish to go with him to get it, but when he arrived at the place, where there was obviously no trace of the organ, the monkey claimed that his liver had been stolen and that he had to find the thieves. When the jellyfish returned to Ryujin and explained why the monkey was not with her, he became so enraged that he hit her until her bones were completely crushed.
Ryūjin shinkō (竜神信仰, "faith of the dragon god") is a form of Shinto religious belief that worships dragons as water kami. It is linked to agricultural rituals, prayers for rain and success for fishermen.
The god has shrines throughout Japan and particularly in rural areas where fishing and rainfall for agriculture are important to local communities.