ABSTRACT: The aim of this study was to clarify the effect of living alone on the cognitive function of older people and the mediating effect of instrumental activities of daily living (IADL) ability.The data for a final sample of 3276 participants aged 65 years and above who did not require long-term care at the baseline were used from a 4-year prospective cohort study conducted in Kasama City, Japan. Demographic data including age, sex, and depression at baseline were used as covariates. The Kihon checklist evaluated the IADL ability at baseline and cognitive function at follow-up. The characteristics of those living alone and with others were compared using the student t test and χ2 test. The effect of living alone on cognitive function was analyzed using logistic regression analysis. Mediation analyses determined the mediating effects of IADL.A total of 325 participants were living alone; they were significantly older than those living with others, more likely to be female, not provide emotional support, and have low physical function, more severe depression, and lower IADL disability. Living alone had a significantly lower risk of cognitive decline at follow-up than living with others. The mediation analysis revealed that IADL disability at follow-up was significantly associated with cognitive decline. Thus, greater IADL ability decreased cognitive decline risk.Older people living alone had a significantly lower risk of cognitive decline, and cognitive function significantly mediated IADL ability. Health support for enhancing IADL abilities may help older people living alone maintain good cognitive function.