Ryūgū-jō or Ryūgū is the supernatural undersea palace of Ryūjin or Dragon God in Japanese tradition.

Ryūjin, which in some traditions is equivalent to Ōwatatsumi, was the tutelary deity of the sea in Japanese mythology. In many versions Ryūjin had the ability to transform into a human shape. Many believed the god had knowledge on medicine and many considered him as the bringer of rain and thunder, Ryūjin is also the patron god (ujigami) of several family groups.