This study was performed to investigate the anti-obesity effect of Eisenia bicyclis in mice fed a high-fat diet (HFD). Male C57BL/6J mice were divided into three groups that were fed a normal diet, an HFD, or an HFD supplemented with a 5% powder of Eisenia bicyclis (PEB) for 8 weeks. The PEB group showed lower body weight gains than the HFD group. The PEB group also exhibited reduced body fat mass and adipose cell size in epididymal adipose tissue. The concentrations of serum cholesterol, leptin, and insulin in the PEB group were significantly lower than those in the HFD group. Liver triglyceride content was significantly decreased by PEB supplementation. Furthermore, hematoxylin and eosin staining revealed that PEB supplementation reduced lipid droplet formation in the liver induced by HFD. These results suggest that PEB supplementation reduces body weight gain and fat accumulation in HFD-induced obese mice.