The risks harmful algal blooms (HAB) pose to aquatic ecosystems, public health, and coastal economies necessitate supplementation of current observation strategies. Herein, we explore the use of red solar induced fluorescence (SIF) from the TROPOspheric Monitoring Instrument (TROPOMI), a remote sensing measurement retrievable in variable cloud conditions, for Karenia brevis detection. Along the West Florida Shelf from 2018 to 2020, we compare red SIF with normalized fluorescence line height (nFLH) from MODIS-Aqua, a standard remote sensing HAB indicator limited to clear sky days. A strong positive linear relationship is found between nFLH and red SIF during severe bloom conditions in 2018 (N = 33,376, r-value = 0.79, R 2 = 0.63) permitting direct comparison. Red SIF provided nearly double the amount of spatiotemporal fluorescence information than nFLH. This work presents the first application of TROPOMI's red SIF for HAB monitoring and illuminates an approach for bolstering early warning systems for HABs beyond clear sky conditions. Plain Language Summary When microscopic algae receive their favorable amount of light and nutrients, they can rapidly multiply and some types of algae have harmful impacts on the environment and humans. As algae photosynthesize, they emit a faint glow in the red wavelengths and this glow, known as fluorescence, is detectable by satellite instruments. We compared two satellite instruments capable of measuring fluorescence, Sentinel 5 Precursor TROPOspheric Monitoring Instrument (TROPOMI) and MODIS-Aqua, during massive harmful algal blooms along the West Florida Shelf from 2018 to 2020. We found that the two instruments were strongly correlated during severe harmful algal blooms and found that TROPOMI was able to gather more fluorescence information in space and time than MODIS-Aqua because it can retrieve fluorescence in optically thin cloud conditions.