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In the present study, a magnetically separable potent solid acid nanocatalyst with a facile preparation technique was fabricated via incorporation of fibroin as a natural polymer with Fe3O4 nanoparticles. In fact, the Fe3O4 nanoparticles were thoroughly covered by the fibroin protein, followed by treating with chlorosulfuric acid for surface sulfonation. Next, physicochemical properties were characterized using FT-IR, XRD, TGA, SEM, EDX, and TEM techniques to elucidate the Fe3O4@fibroin-SO3H structure. The analyses proved the core–shell structure and the presence of sulfonyl on the surface. The Fe3O4@fibroin-SO3H showed significant activity in the tetrazole derivatives synthesis reactions under solvent-free and relatively mild conditions. The structure of the products was validated using 1H-NMR and 13C-NMR. The prepared catalyst showed good stability and reusability even after easy separation by an external magnet for four consecutive reaction rounds. Considering the crucial importance of sulfuric acid catalysis, we believe that Fe3O4@fibroin-SO3H will find wide applications as a separable, green, and potent solid acid nanocatalyst. [ABSTRACT FROM AUTHOR]