Seong Hae-eung was a scholar in late eighteenth- and early nineteenth-century Korea, who left a vast amount of scholarly works. One of his writings, the Yeon-gyeongjae oejib 硏經齋外集, includes a manuscript, titled “Jogang doji 祖江濤志”. In this, Seong discussed the tidal times and the origin of tidal occurrence in the Jogang 祖江 River, the junction of the Han 漢江 and Imjin 臨津江 Rivers. Seong thought that tidal movements were influenced by the movements of sun and moon. He believed that tides originated in the “yanghai” 陽海 ocean of the southeastern part of the world. In yanghai, water was hot and stirring, and thus was transformed into waves; this was the origin of the tide. Tidal movements would thus show regional variations, depending upon the qualities of yin and yang forces at specific regions. Seong Hae-eung’s tidal theory, based on the yin and yang theory, was also an elaboration of the tidal theory of Sin Gyeongjun 申景濬, a Confucian scholar and geographer in the eighteenth century. In order to create a tide table of the Jogang River, Seong started from Sin’s research, investigated various discussions on tides by previous scholars in China and Korea, and finally interpreted those theories in his own way. Seong’s article could thus be considered one of the important works in the development of the tidal theory in late Joseon period.