The obesity rate has been increasing worldwide, which is important because obesity has been linked to the development of various metabolic disorders, such as type 2 diabetes, hypertension, cancer, and stroke. Nanomedicine offers a new approach for treating many diseases, including metabolic disorders such as obesity. In this study, we explored the anti-adipogenic effects of spherical gold nanoparticles synthesized with fresh Panax ginseng leaves (P.g AuNPS) in vitro using 3T3-L1 mature adipocytes. Cell viability was assessed by quantitating preadipocyte growth at different time points. Furthermore, to assess the anti-adipogenic effects of P.g AuNPS, intracellular lipid accumulation was investigated in mature adipocytes. To this end, cells were observed under a microscope and OD measurements were taken after Oil Red O staining. In addition, transcriptional gene regulation was examined by performing real time PCR to assess the levels of adipogenic genes such as PPARγ, CEBPα, CEBPβ, Jak2, STAT3, FAS, SREBP-1, and ap2. Moreover, protein levels were evaluated by immunoblotting. Altogether, these results confirm that P.g AuNPS exhibit anti-adipogenic effects at a concentration of 100 μg/ml and that these effects are mediated by the downregulation of PPARγ/CEBPα (major transcription factors) signaling in 3T3-L1 mature adipocytes.