Ryūjin, the Japanese dragon god of the sea, rules from Ryūgū-jō — a magnificent undersea palace built from red and white coral, where time itself does not move at the same pace it does on land. That single detail about time is the hinge on which Japan's most famous story about him turns.
Ryūjin is best understood as Japan's assimilation of the Chinese Dragon King concept, layered onto — and eventually largely merged with — an older native Shinto sea deity known as Watatsumi. This is a genuinely traceable case of religious and mythological transmission: as Chinese Buddhist and Daoist dragon imagery spread into Japan, it fused with existing indigenous sea-god worship rather than simply replacing it, producing the figure known today interchangeably as Ryūjin or Watatsumi depending on the source and era.
In Japan's best-known folktale involving Ryūjin's undersea realm, a kind-hearted fisherman named Urashima Tarō rescues a turtle from mistreatment by village children. In gratitude, the turtle carries him down to Ryūgū-jō, where he is welcomed by Ryūjin's daughter, the beautiful princess Otohime. Urashima stays at the palace for what feels like only a few pleasant days, but when he finally asks to return home to check on his aging parents, Otohime gives him a mysterious box, the tamatebako, with a single instruction: never open it.
Urashima returns to the surface to find his village unrecognisable — centuries, not days, had passed while he enjoyed the timeless palace beneath the waves. Everyone he knew is long dead. In grief and confusion, he opens the forbidden box despite Otohime's warning, and a cloud of white smoke pours out — instantly aging him into an old man, the years he never lived on land catching up with him all at once.
A meditation on time's asymmetry: Urashima Tarō is not really a cautionary tale about disobedience — it is a story about the incompatibility of paradise and ordinary human time. The tamatebako doesn't punish him for curiosity so much as it simply releases the years his body was owed all along, the moment he steps back into the world that kept moving without him.
The imperial connection: a related, older myth from the Kojiki tells of Hoori (Yamasachihiko), who visits the sea god's palace and marries the sea deity's daughter, Toyotama-hime. When she is about to give birth, she asks Hoori never to look at her during labour, retreating into a birthing hut and taking her true form. Overcome with curiosity, he looks anyway — and sees she has become a wani, a sea-monster or dragon-like creature. Mortified at being seen, she abandons both Hoori and their newborn son and returns permanently to the sea, sealing an enduring boundary between the human and sea-deity worlds. That son, raised instead by Toyotama-hime's sister, becomes the father of Emperor Jimmu — the legendary first emperor of Japan — making the dragon-sea-god lineage a literal, if mythological, ancestor of the entire imperial line.