To evaluate the anti-obesity effects of Smilax sieboldii (Smilacaceae), we screened methylene chlo-ride (CH2Cl2), ethyl acetate (EtOAc), aqueous-saturated n-butanol, and ethanol (EtOH) extracts of the whole plant at various concentrations to inhibit adipogenesis in adipocytes. The 3T3-L1 cell line with Oil red O staining with the help of fluorometry was used as an indicator of anti-obesity ac-tivity. Bioactivity-guided fractionation of the EtOH extract and subsequent phytochemical inves-tigation of the active CH2Cl2- and EtOAc-soluble fractions resulted in the isolation of 19 secondary metabolites (1–19), including a new α-hydroxy acid derivative (16) and two new lanostane-type triterpenoids (17 and 18). The structures of these compounds were characterized using various spectroscopic methods. All the isolated compounds were screened for adipogenesis inhibition at concentration of 100 μM. Of these, compounds 1, 2, 4-9, 15, and 19 significantly reduced fat ac-cumulation in 3T3-L1 adipocytes, especially compounds 4, 7, 9, and 19 showing 37.05 ± 0.95, 8.60 ± 0.41 15.82 ± 1.23, and 17.73 ± 1.28% lipid content, respectively, at a concentration of 100 μM. These findings provide experimental evidence that isolates from S. sieboldii extracts exert beneficial ef-fects regarding the regulation of adipocyte differentiation.