A subtropical small jellyfish, the blue umbrella jellyfish (diameter 2~3 cm), has been first observed in the waters off Jeju Island and has since spread to the southern and eastern coasts, necessitating special attention. The National Institute of Fisheries Science revealed that since the blue umbrella jellyfish was first discovered in the waters off Jeju Island on July 17, it has been appearing in large numbers in areas along the southern and eastern coasts, including Jeonnam, Gyeongnam, Busan, and Gyeongbuk, and is expected to spread to the coastal areas of Gangwon. This situation was confirmed through field surveys in Jeju (July 28 to August 2) and citizen participation mobile web reports (July 17 to August 4, 2025). The blue umbrella jellyfish has low toxicity and does not cause significant damage to fisheries.