From 2017 to 2024, we surveyed 43 diverse aquatic habitats in South Korea, leading to the identification of 18 cyanobacterial taxa that are newly recorded for the country, found across eight sites (about 18% of the surveyed locations). These taxa exhibit a wide range of morphological forms, including unicellular, colonial, filamentous, and heterocytous types, and belong to various orders such as Chroococcales, Synechococcales, Nostocales, and Stigonematales. Notably, this study provides a provisional record of Gomphosphaeria aponina in Korea, correcting its previous misidentification as G. natans. We also documented Dolichospermum compactum, a species that has been genetically reclassified. Additionally, we identified species with the potential to cause harmful algal blooms (HABs), such as Microcystis botrys and Gloeotrichia aurantiaca, which are crucial for domestic water quality monitoring. Currently, only 414 cyanobacterial taxa are recorded in Korea, representing less than 8% of the estimated global total of approximately 5,300 species. This significant gap underscores the considerable unrecorded diversity within Korean aquatic ecosystems. These findings substantially enhance the national cyanobacterial checklist and underscore the need for ongoing monitoring in understudied aquatic environments. They also highlight the importance of integrating classical morphological and ecological observations with advanced molecular methods. This polyphasic approach can accurately detect cryptic diversity and support robust ecological assessments. Overall, this comprehensive floristic expansion offers valuable baseline data for biodiversity inventories, ecological monitoring, and the development of microbial resources within Korean aquatic environments.