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Background With global population ageing, frailty poses a growing public-health challenge. Many countries have promoted social participation to mitigate frailty among older adults, but the cross-national consistency of this association remains unclear.Methods We harmonised data from four longitudinal cohort studies (N=20 947) conducted in England, the USA, China and Mexico with respect to measures of social participation and frailty. We employed fixed-effects models and cross-national meta-analyses to examine the longitudinal relationship between social participation and frailty. The Arellano-Bond estimator model was further used to evaluate the lagged effects of this association. Finally, a multilevel analysis incorporating country-level factors was conducted to investigate the moderating effects of macro-level variables.Results Substantial differences in social participation were observed across countries, ranging from 41.76% in Mexico to 83.43% in the USA. After adjusting for confounders, higher social participation was significantly associated with lower frailty levels (pooled effect=−0.52; 95% CI −0.80 to −0.24), with evidence of a lagged temporal effect. Neither the strength of this association nor the overall rate of social participation was significantly influenced by country-level factors.Conclusion The consistent findings across diverse national contexts suggest that promoting equitable social engagement among older adults may effectively reduce the prevalence of frailty. Public-health policies and community programmes should prioritise initiatives that encourage social participation among the elderly, and healthcare services may consider integrating social activities into routine care to foster active ageing.