To examine the effect of social support on the risk of caregiver's functional disability due to spouse's functional disability.Longitudinal study.In this cohort study using the Ohsaki Cohort 2006 Study data, the baseline survey was conducted from December 1, 2006, to December 15, 2006, and included 7598 older adults, including 3799 couples, ≥65 years of age.The incidence of spouse's functional disability was defined as certification for the Long-term Care Insurance, which uses a nationally uniform standard of functional disability. The primary outcome was the incidence of the caregiver's functional disability, and the follow-up period was between December 16, 2006, and November 30, 2019. Five social support questionnaires were used to assess the degree of emotional and instrumental social support available to each participant. The Cox proportional hazards model was used to evaluate the incidence of functional disability among caregivers after the occurrence of spouses' functional disability. Subgroup analyses were also conducted according to the status of emotional and instrumental social support.The caregiver's functional disability increased significantly among those whose spouses had functional disability compared with those whose spouses had no disability [multivariate hazard ratio (HR), 1.86]. Emotional and instrumental social support showed significant positive interactions on this association (emotional and instrumental support: P for interaction < .01 and < .01, respectively), and the risk of caregiver's disability was higher among those without social support than among those with social support (with emotional support: HR, 1.84; without emotional support: HR, 2.51; with instrumental support: HR, 1.85; without instrumental support: HR, 2.31).Social support may help to alleviate the increased risk for caregiver's functional disability due to the spouse's functional disability.