Cathepsin C (CatC), a key enzyme in neutrophil serine protease activation, is a promising target for treating neutrophilic inflammatory diseases like acute lung injury, ARDS, and COVID-19. Despite its therapeutic potential, no CatC inhibitors are currently available. In this study, a series of gallotannin derivatives were isolated from the traditional Chinese medicine Rhois Galla. Among these, 1,2,3,6-tetra-O-galloyl-β-d-glucose (1) inhibited CatC with an IC