Organo-Se species represent a type of increasingly important core motif widely found in endogenous proteins and bioactive small molecules. Consequently, exploring their structural diversity to entrust their powerful potential has emerged as an important endeavor in basic research, but it remains extremely limited and highly challenging. Here, we realize either racemic or asymmetric synthesis toward a class of organo-Se species, namely benzothiaselenazole-1-oxides (BTSAs), and reveal its diversified applications. For example, we identified that BTSAs can be recognized as the click selenylation reagents, which not only enabled on-plate modular construction of parallel (1,066 examples) and DNA-encoded (261 examples) indole-C₃-selenide libraries but also provided the site-specific modification of sulfhydryl-containing chemicals for application in anti-COVID-19 drug discovery and bioorthogonal labeling-based HER2 fluorescence imaging analysis. This work greatly expands the space of organo-Se chemistry, and given the potent performance of BTSAs in click selenylation, it should find robust application in chemistry-related research areas.