In East Asia, dragons are beings with supernatural and mystical powers and have often been objects of worship and belief. In Korea, there are numerous records and legends about dragons since ancient times, and dragons are the highest deity that governs water, and have become established as a folk religion that prays for water-related wishes such as a good catch or prayer for rain. Most of the assertions made in existing research were that Koreans had their own unique watery deity beliefs that had been passed down since ancient times, but with the introduction of Buddhism, indigenous beliefs were diminished or replaced by Buddhist beliefs. Although it is true that there have been many changes in the indigenous watery deity belief with the importation of the Chinese dragon god and the Buddhist dragon king concept, it can be said that the original body and identity of the indigenous watery deity still remain in the folk watery deity belief. In this context, this paper attempted to reveal the identity of Korea's indigenous watery deity. To this end, first, by comparison of the origins and histories of the dragon concepts in Korea and China, the forms of watery deity beliefs in folk religions were analyzed. The analysis result revealed that turtles have been believed and revered as dragon kings in Korean folk religion, and that Maitreya belief flourished greatly from the Silla period. This can be said to be a characteristic of the Korean watery deity and shows a dual structure of Korean belief in the dragon king.