Soybean-derived phosphatidylserine (Soy-PS) is a phosphatidylserine made from soybean lecithin by enzymatic reaction with L-serine. A double-blind, randomized controlled study was conducted to investigate the effects of Soy-PS on the cognitive functions of the elderly Japanese subjects with memory complaints. Seventy-eight elderly people with mild cognitive impairment (50–69 years old) were randomly allocated to take Soy-PS (100 mg, 300 mg/day) or placebo for 6 months. As a result, there was no difference in blood markers and vital signs during Soy-PS treatment and any side effect caused by Soy-PS treatment was not observed. Neuropsychological test scores were similarly increased in all groups including placebo group. However, in the subjects with relatively low score at baseline, the memory scores in PS treated groups were significantly increased against the baseline, while those of placebo group remained unchanged. And the memory improvements in Soy-PS-treated groups were mostly attributed to the increase in delayed verbal recall, a memory ability attenuated in the earliest stage of dementia. In conclusion, Soy-PS used in this study is considered as safety food ingredient and 6 months of Soy-PS supplementation could improve the memory functions of the elderly with memory complaints.