OBJECTIVE To explore the anti-obesity effects and the mechanism of action of Monascus pilosus(M. pilosus)-fermented black soybean (MFBS) extracts (MFBSE) and MFBS powders (MFBSP) in adipocytes and high-fat diet (HFD)-induced obese mice, respectively. METHODS Black soybean was fermented with M. pilosus, and the main constituents in MFBS were analyzed by HPLC analysis. In vitro, MFBSE were examined for anti-adipogenic effects using Oil-Red O staining. In vivo, mice were fed a normal-fat diet (NFD) control, HFD control or HFD containing 1 g/kg MFBSP for 12 weeks, and then body weight gain and tissues weight measured. Real-time PCR and western blot assay were used to determine the mechanism of anti-adipogenic effects. RESULTS MFBSE inhibited lipid accumulation in 3T3-L1 adipocytes without exerting cell cytotoxicity. MFBSP treatment in HFD-fed mice significantly decreased the body weight gain compared with the HFD control mice. MFBSE and MFBSP treatment resulted in significantly lower mRNA levels of adipogenesis-related genes, such as peroxisome proliferator-activated receptor γ(PPAR γ), fatty acid-binding protein 4 (FABP4), and fatty acid synthase (FAS), in adipocytes and in white adipose tissue (WAT) of HFD-induced obese mice. CONCLUSIONS These results suggest that the anti-obesity effects of MFBS are elicited by regulating the expression of adipogenesis-related genes in adipocytes and WAT of HFD-induced obese mice.