In this study, we investigated the antiobesity properties of Petalonia binghamiae extract (PBE) in mice in which obesity was induced with a high-fat diet (HFD). PBE administration (150 mg/kg/day) for 70 days decreased body weight gain, adipose tissue weight, and the serum triglyceride level in mice fed a HFD. PBE reduced serum levels of glutamic pyruvic transaminase and glutamic oxaloacetic transaminase as well as the accumulation of lipid droplets in the liver. PBE restored the HFD-induced decrease in phosphorylation of AMP-activated protein kinase (AMPK) and acetyl-CoA carboxylase (ACC) in epididymal adipose tissue. PBE increased the phosphorylation of AMPK and ACC and decreased the expression of SREBP1c in mature 3T3-L1 adipocytes. In addition, we further explored the active compound responsible for AMPK activation by PBE in 3T3-L1 adipocytes. Fucoxanthin isolated from PBE increased the phosphorylation of AMPK and ACC with increasing LKB1 phosphorylation in mature 3T3-L1 adipocytes. Taken together, these data suggest that PBE (or fucoxanthin) exert improving effects on HFD-induced obesity by promoting β-oxidation and reducing lipogenesis.