Obesity is the main clinical manifestation of the disturbance of energy intake and consumption, and is a significant risk factor for diabetes mellitus, hypertension, dyslipidemia, and cardiovascular disease. In the present study, Sargassum hemiphyllum ethanolic extract (SHEE) containing 7.56 mg of fucoxanthin per gram of dry matter was investigated to determine its potential in inhibiting adipogenesis in adipocytes 3T3-L1 cells. SHEE at concentrations of 250, 500, and 750 μg/ml had no cytotoxic effect on 3T3-L1 cells. When cells were treated with 750 μg/ml SHEE, lipid accumulation was significantly reduced to 1.26 ± 0.06 mM compared with control cells (1.62 ± 0.05 mM). In relation to adiponectin activation, secretion and mRNA expression were significantly increased at 500 and 750 μg/ml SHEE. In addition, 3T3-L1 cells treating with 750 μg/ml SHEE significantly decreased mRNA expression of peroxisome proliferator-activated receptor γ (PPARγ) 0.44 ± 0.09-fold compared with the control. These results suggest that fucoxanthin-containing SHEE inhibited adipogenesis in 3T3-L1 cells through downregulation of PPARγ.