The anti-obesity potential of an ethanolic extract of the edible red alga Callophyllis japonica extract (CJE) was investigated in mice fed a high-fat diet (HFD). CJE administration into HFD mice revealed suppression of body weight, adipose tissue weight, serum total cholesterol, triglyceride, and glucose levels in a dose-dependent manner. Also, it reduced serum levels of glutamic pyruvic transaminase, glutamic oxaloacetic transaminase, and lactate dehydrogenase, as well as the accumulation of fatty droplets in liver tissue. CJE and its ethyl acetate fraction inhibited adipogenesis in 3T3-L1 adipocytes by down-regulating the adipocyte-specific transcriptional regulators. Taken together, these results suggest that CJE reduces obesity in mice fed an HFD by inhibiting lipid accumulation and adipogenesis in the adipose tissues.