Saponins are a diverse group of biologically functional products in plants. Soyasaponins are usually glycosylated, which give rise to a wide diversity of structures and functions. In this study, we investigated the effects and molecular mechanism of soyasaponins Aa and Ab in regulating adipocyte differentiation and expression of adipogenic marker genes in 3T3-L1 adipocytes. Soyasaponins Aa and Ab dose-dependently inhibited the accumulation of lipids and the expression of adiponectin, adipocyte determination and differentiation factor 1/sterol regulatory element binding protein 1c, adipocyte fatty acid-binding protein 2, fatty acid synthase, and resistin in 3T3-L1 adipocytes. In addition, soyasaponins Aa and Ab suppressed the transcriptional activity of peroxisome proliferator-activated receptor γ (PPARγ) in HEK 293T cells. Furthermore, we confirmed that the expression of PPARγ and of CCAAT-enhancer-binding protein α (C/EBPα) was suppressed at both the mRNA and protein levels in 3T3-L1 adipocytes by treatment with soyasaponins Aa and Ab. Taken together, these findings indicate that soyasaponin Aa and Ab markedly inhibit adipocyte differentiation and expression of various adipogenic marker genes through the downregulation of the adipogenesis-related transcription factors PPARγ and C/EBPα in 3T3-L1 adipocytes.