Background Diabetic nephropathy (DN) is the leading cause of end-stage renal disease. Various treatment regimens and combinations of therapies provide only partial renoprotection. Therefore new approaches are needed to retard the progression of DN. The aim of the present study was to evaluate the role of a novel spiroalkaloid from Acorus tatarinowii named acortatarin A (AcorA) in inhibiting high glucose-induced extracellular matrix accumulation in mesangial cells (MCs). Methods The cytotoxity of AcorA on MCs was examined by 3-(4,5-dimethylthiazol-2-yl)-2.5-diphenyltetrazolium bromide (MTT) assay. The expression of fibronectin and collagen IV was examined by real time PCR and western blotting. The expression of p22phox and p47phox was detected by western blot. The interaction between p22phox and p47phox was examined by co-immunoprecipitation. The phosphorylation of p47phox was examined by immunoprecipitation. The phosphorylation of protein kinase C (PKC) α, PKCβ, phospholiase C gamma (PLCγ1), and the p85 subunit of PI3K was determined by Western blotting. Results AcorA significantly inhibited high glucose-induced activation of NADPH oxidase, a ROS-generating enzyme, by increasing phosphorylation of p47phox and enhancing interaction between p22phox and p47phox. Preincubation of AcorA with MCs inhibited high glucose-induced collagen IV and fibronectin production in a dose-dependent manner. Moreover, AcorA attenuated high glucose enhanced phosphorylation of PKCα, PKCβ, PLCγ1, and the p85 subunit of PI3K. Conclusion AcorA inhibits high glucose-induced extracellular matrix production via blocking NADPH oxidase activation.