Although various treatments have been used for weight loss to date, obese people rarely have safe and effective treatment options. Therefore, the antiobesity effects of several natural compounds are being actively investigated. This study was conducted to investigate the antiadipogenic effects of Monascus ruber-fermented Fagopyrum esculentum (red yeast buckwheat, RYB) in 3T3-L1 cells. We assessed the intracellular lipid content and adipocyte differentiation by oil red O staining and the expression of genes and proteins associated with adipocyte differentiation by reverse transcription-polymerase chain reaction and western blotting in 3T3-L1 cells. RYB dose dependently inhibited 3T3-L1 cell differentiation at concentrations of 50-800 lg/mL, without cytotoxic effects. It also suppressed the expression of adipogenic transcription factors, including peroxisome proliferator-activated receptor gamma and CCAAT/enhancer-binding protein a, and adipocytespecific genes, such as adipocyte fatty acid-binding protein (aP2), fatty acid synthase, and leptin, during preadipocyte differentiation into adipocytes. Furthermore, RYB reduced cyclin-dependent kinase 2 and cyclin expression and increased p21 and p27 expression, thus causing cell cycle arrest at the G1/S phase. Collectively, these results suggest that RYB may be an effective nutraceutical for weight loss as indicated by its ability to suppress adipogenesis-specific gene expression and cause cell cycle arrest at the G1/S interphase.