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[Display omitted] • Nanofiltration membranes with strong negative charge were prepared by a new aqueous monomer 2,4,6-trimethyl-3,5-diaminobenzenesulfonamide (TMDBSA) through the traditional interfacial polymerization process without any post-treatment. • The membrane has high water permeability (19.6 L m−2 h−1 bar−1) and >98.3% methyl blue rejection. • It owns separation stability for the removal of MB in a harsh acid environment. • It has excellent antifouling performance and long-term stability during the filtration tests. • It demonstrates great practical application potential for treating acidic anionic dyes solutions. Nanofiltration (NF) membranes with negative surface charge are of critical significance for the efficient removal of anionic dyes from wastewater. In this work, a novel negatively charged NF membrane was prepared from a new sulfonamide monomer, 2,4,6-trimethyl-3,5-diaminobenzenesulfonamide (TMDBSA) and trimesoyl chloride (TMC) via interfacial polymerization. The unique diamino-sulfonamide structure of TMDBSA would be conducive to the formation of a poly (amide-sulfonamide) (PASA) layer with enhanced negative charge while maintaining the appropriate degree of crosslinking. The TMDBSA-TMC membrane prepared at optimized condition exhibits both high water permeability (19.6 L m−2h−1 bar−1) and superior rejections of anionic dyes with different molecular weights (e.g. 98.3% rejection for methyl blue), reveals that Donnan exclusion dominates the rejection mechanism. More importantly, benefiting from the strong negatively charged surface and PASA structure, the TMDBSA-TMC membrane shows separation stability for the removal of methyl blue in a broad pH range, even under a harsh acid environment. Meanwhile, the membrane has excellent antifouling performance and long-term stability during the filtration tests. The newly developed negatively charged NF membrane demonstrates great practical application potential for the removal of anionic dyes with small molecular weights. [ABSTRACT FROM AUTHOR]