Radiosumin, a novel potent trypsin inhibitory dipeptide, was isolated from the freshwater blue-green alga Plectonema radiosum (NIES-515). Its structure was elucidated to be 1 on the basis of 2D NMR data and chemical degradation. The IC50 of 1 against trypsin was 0.14 µg/mL. In recent years, blue-green algae have been demonstrated to be valuable sources of unique peptidic compounds, such as microviridin,1 hormothamnin A,2 calophycin,3 microginin,4 micropeptins,5 and aeruginosins.6 In the course of our screening program of protease inhibitors from microalgae, we have found that the aqueous layer from cultures of the freshwater blue-green alga Plectonema radiosum (NIES-515) strongly inhibited trypsin. We isolated radiosumin (1), a novel potent trypsin inhibitory dipeptide, and elucidated the structure to be 1 on the basis of 2D NMR data and chemical degradation. The IC50 of 1 against trypsin was 0.14 µg/mL. We herein report the isolation and structural determination of 1.