Soy (Glycine max, family Leguminosae), which contains isoflavones and saponins as main constituents, is known to exhibit memory-enhancing effects. Therefore, to investigate the role of soyasaponins in memory impairments, we isolated soyasaponins Ab (SA) and Bb (SB) from soybean and measured their protective effects against scopolamine-induced memory impairment in mice. SA and SB significantly prevented scopolamine-induced memory impairment in passive avoidance and Y-maze tasks. Compared to SA, SB rescued memory impairment more potently. Treatment with SB (10 mg/kg, p.o.) protected memory impairment in passive avoidance and Y-maze tasks to 97% (F = 68.10, P < 0.05) and 78% (F = 35.57, P < 0.05) of untreated normal control level, respectively. SA and SB (10 mg/kg) also rescued scopolamine-induced memory impairment in Morris water maze task (F = 14.51, P < 0.05). In addition, soyasaponins preserved brain-derived neurotrophic factor (BNDF) expression (F = 33.69, P < 0.05) and cAMP response element-binding (CREB) protein phosphorylation (F = 91.62, P < 0.05) in the hippocampus of scopolamine-treated mice. However, SA and SB did not inhibit acetylcholinesterase in vitro and ex vivo. On the basis of these findings, we suggest that soybean, particularly soyasaponins, may protect memory impairment by increasing BDNF expression and CREB phosphorylation.