A severe red tide event, caused primarily by dinoflagellate Karenia selliformis, occurred during the autumn of 2021 in the waters off southeast Hokkaido and resulted in damage of $70 million to fishery industries. There is a high demand for early warning methods based on ocean colour observations to respond to future occurrences of red tides. We therefore used a quasi-analytical algorithm to compute the total absorption coefficient (a) from the Second Generation Global Imager (SGLI) remote sensing reflectance and assessed its potential to detect blooms of K. selliformis. We discovered that, within the same range of SGLI-retrieved chlorophyll-a concentrations, the a at a wavelength of 530 nm a(530) tended to be lower during K. selliformis blooms than during diatom blooms. The a(530) observed by the SGLI therefore showed promise as an optical property for detecting K. selliformis blooms. We discuss the reasons for selection of a(530) and the limitations of the current study's bloom detection method. Red tide caused by K.selliformis in the waters off southeast Hokkaido resulted in damage of $70 million to fishery industries Remote sensing-based observation that can serve as an early warning for red tide occurrence is thus required by fishery-related stakeholders We therefore investigated a remote sensing-based algorithm for detecting K.selliformis blooms in the waters off southeast Hokkaido [ABSTRACT FROM AUTHOR]