This study explored the association of marital transitions and frailty among Chinese middle-aged and older people and whether this association differs by social support.We used a sample of 12,388 adults aged ≥45 years who participated in the China Health and Retirement Longitudinal Study (CHARLS) between 2015 and 2018. Between-wave changes in marital status ("married at both times", "unmarried to married", "married to unmarried", "unmarried at both times") were used to explore the changes in frailty measured by the frailty index (FI), which was constructed from 55 health variables. Social support was evaluated based on social engagement and intergenerational support. The associations among marital transitions, social support and frailty were analyzed using generalized estimating equations (GEEs).The mean FI of 12,388 participants was 0.23 (SD = 0.13). Participants who were married to unmarried (β = 0.014, B = 0.005, P = 0.012) and unmarried at both times (β = 0.022, B = 0.003, P < 0.001) had significant a positive impact on FI compared with participants who were married at both times. Social engagement, financial support by children and providing care to grandchildren had an interactive effect with marital transitions in influencing FI.Being unmarried may increase frailty among Chinese middle-aged and older adults. Financial support by children may mitigate the adverse effects of being unmarried on frailty.© 2023 The Authors. Published by Elsevier Ltd.