Obstructive nephropathy is the end result of a variety of diseases that block drainage from the kidney(s). Transforming growth factor-β1 (TGF-β1)/Smad3-driven renal fibrosis is the common pathogenesis of obstructive nephropathy. In this study, we identified petchiether A (petA), a novel small-molecule meroterpenoid from Ganoderma, as a potential inhibitor of TGF-β1-induced Smad3 phosphorylation. The obstructive nephropathy was induced by unilateral ureteral obstruction (UUO) in mice. Mice received an intraperitoneal injection of petA/vehicle before and after UUO or sham operation. An in vivo study revealed that petA protected against renal inflammation and fibrosis by reducing the infiltration of macrophages, inhibiting the expression of proinflammatory cytokines (interleukin-1β and tumour necrosis factor-α) and reducing extracellular matrix deposition (α-smooth muscle actin, collagen I and fibronectin) in the obstructed kidney of UUO mice; these changes were associated with suppression of Smad3 and NF-κB p65 phosphorylation. Petchiether A inhibited Smad3 phosphorylation in vitro and down-regulated the expression of the fibrotic marker collagen I in TGF-β1-treated renal epithelial cells. Further, we found that petA dose-dependently suppressed Smad3-responsive promoter activity, indicating that petA inhibits gene expression downstream of the TGF-β/Smad3 signalling pathway. In conclusion, our findings suggest that petA protects against renal inflammation and fibrosis by selectively inhibiting TGF-β/Smad3 signalling. © 2019 The Authors. Journal of Cellular and Molecular Medicine published by John Wiley & Sons Ltd and Foundation for Cellular and Molecular Medicine.