Background Retirement is a major life-course transition that may reshape health, social participation, and economic security in later life. The Active Ageing Index captures these domains, yet longitudinal evidence on how active ageing changes after retirement remains limited. This study examined post-retirement trajectories of active ageing among older adults in Korea.Methods Five biennial waves (2014–2022) of the Korean Longitudinal Study of Aging were analyzed. Individuals who transitioned from employment to retirement were identified and matched to continuously employed counterparts using propensity score matching to balance pre-retirement sociodemographic and health-related characteristics. Changes in Active Ageing Index scores around retirement were estimated using comparative interrupted time series models. Cox proportional hazards models were applied to examine factors associated with transition to low active ageing. All models adjusted for baseline covariates including age, sex, education, income, and health status.Results Retirement was associated with a significant immediate decline in overall Active Ageing Index scores (β = −0.0914, p < 0.0001), but no sustained post-retirement downward trend was observed. Regional differences did not significantly modify post-retirement trajectories, as the interaction between retirement status and region was not statistically significant (β = −0.0002, p = 0.9872). In survival analyses, adults aged 75 years or older had a higher risk of transitioning to low active ageing (HR: 1.350, 95% CI: 1.025–1.778), whereas regular physical activity was associated with a lower risk (HR: 0.697, 95% CI: 0.559–0.868).Conclusions Retirement represents a critical transition characterized by short-term disruption in active ageing rather than sustained decline. Interventions that promote physical activity and continued engagement during the retirement transition may be more effective than region-based approaches in supporting active ageing among older adults.