The present study investigated the antiobesity effect of Achyranthes bidentata Blume root water extract in a 3T3-L1 adipocyte differentiation model and rats fed with a high-fat diet. To investigate the effect of Achyranthes bidentata Blume on adipogenesis in vitro, differentiating 3T3-L1 cells in adipocyte-induction media were treated every two days with Achyranthes bidentata Blume at various concentrations (1 to 25 μg/mL) for eight days. We found that Achyranthes bidentata Blume root inhibited 3T3-L1 adipocyte differentiation without affecting cell viability, and Western blot analysis revealed that phospho-Akt expression was markedly decreased, whereas there was no significant change in perilipin expression. Furthermore, administration of Achyranthes bidentata Blume root (0.5 g/kg body weight for six weeks) to rats fed with a high-fat diet significantly reduced body weight gain without affecting food intake, and the level of triglyceride was significantly decreased when compared to those in rats fed with only a high-fat diet. These results suggest that Achyranthes bidentata Blume root water extract could have a beneficial effect on inhibition of adipogenesis and controlling body weight in rats fed with a high-fat diet.