This study provides one of the first population-based investigations of the longitudinal association between social isolation and sleep difficulty among older adults in China. We analyzed three waves of longitudinal data from the China Longitudinal Aging Social Survey (2014-2018), in which 8456 respondents contributed 16,156 person-year observations. Results from multilevel logistic regression models showed that social isolation was related to a higher risk of sleep difficulty. We also found that socially isolated older adults were more likely to report higher levels of depressive symptoms, a greater prevalence of loneliness and pain, and more chronic diseases compared to their socially integrated counterparts, which in turn increased their risks of sleep difficulty. Moreover, socially isolated older adults with chronic diseases were particularly vulnerable to the risk of sleep difficulty. These findings provide helpful guidance for policymakers and practitioners to design effective intervention strategies to help older adults with sleep problems.