Saikosaponin-A (SA), a class of native compound with numerous biological activities, may exert protective effect against postmenopausal bone loss. However, it remains unknown whether SA regulates the osteogenic differentiation of bone marrow stromal cells (BMSCs) in the treatment and prevention of osteoporosis. In this study, BMSCs were treated with various concentrations of SA to stimulate osteogenic differentiation over a 14-day period. Additionally, a canonical ovariectomized (OVX) mouse model was used to evaluate the effect of 3-month SA treatment in preventing postmenopausal osteoporosis. In vitro, we found that SA promotes alkaline phosphatase activity/staining and Alizarin red assay, stimulated the expression of osteogenic markers, i.e., runt-related transcription factor 2 (Runx2), osterix, osteopontin, and osteocalcin (OCN) in BMSCs. In vivo, the trabecular number, trabecular thickness, and trabecular bone mineral density of the distal femoral metaphysis were significantly increased in OVX mice treated intraperitoneally with SA for 3 months compared with OVX mice that not treated with SA. Moreover, the expression of Runx2 and OCN in OVX + SA mice was significantly increased than that in OVX mice. Finally, we found that SA activated the WNT/beta-catenin pathway and the expression of several downstream genes including T-cell factor-1 and lymphoid enhancer factor-1. Inhibition of WNT/beta-catenin pathway by Dickkopf-related protein 1 blocked the positive role of SA on osteogenesis. Therefore, SA promoted the osteogenic differentiation of BMSCs through WNT/beta-catenin signaling.