The efficacy of cognitive stimulation therapy (CST) in patients with vascular cognitive impairment has not been explored, and no studies investigating CST in the convalescent rehabilitation phase have been reported. This study examined the effect of CST on the cognitive function of patients with vascular cognitive impairment. A randomized controlled, assessor-blinded, single-centered trial with two parallel groups was conducted in a convalescent rehabilitation hospital. Twenty participants were randomly allocated to CST (n=10) and control (n=10) groups. Participants in the CST group underwent two CST sessions a day, five times a week for 8 weeks, in addition to conventional rehabilitation. Participants in the control group underwent conventional rehabilitation only. The primary outcome was the Mini-Mental State Examination (MMSE) score, and the outcome between the groups was compared using a generalized linear mixed model (GLMM). The mean (standard deviation) scores of MMSE increased by 3.50 (3.08) points and 4.50 (1.61) points from baseline to the end of the study (week 8) in the CST and control groups, respectively. The GLMM showed a significant effect of TIME on MMSE (F=21.121, P<0.001), whereas no significant effect on MMSE was observed for GROUP (intervention vs. control, P=0.817) or the interaction term (TIME×GROUP, P=0.649). Although a significant improvement in cognitive function was observed in each group, no significant effect of CST was evident. This result indicates that the effect may have been masked by improvements caused by natural history or rehabilitation. Future studies with a sufficient sample size are required to confirm the findings.