BACKGROUND: We investigated the association between changes in physical activity and the risk of a major adverse cardiovascular event (MACE) in people with newly diagnosed diabetes. METHODS: Using a nationwide database, we identified 8,596 people with newly diagnosed diabetes who underwent national health examinations within a year before and after a diabetes diagnosis. Cox proportional hazards models, hazard ratios (HRs) and 95% confidence intervals (CIs) for MACE risks were calculated according to changes in physical activity before and after a diagnosis of diabetes. RESULTS: During a median follow-up of 2.3 years, study participants who engaged in sustained physical activity after a diagnosis of diabetes had a 34% lower MACE risk compared to those with sustained inactivity (HR, 0.66; 95% CI, 0.44-0.98). An advantage was observed in those with a history of cardiovascular disease, although this was of borderline statistical significance (HR, 0.63; 95% CI, 0.40-1.01; P=0.054). In people considered obese, physical activity was significantly associated with a decreased risk of a MACE, regardless of the period preceding and following the diabetes diagnosis. Those who became inactive to active had the lowest risk of a MACE (HR, 0.38; 95% CI, 0.18-0.79). CONCLUSION: Maintaining active physical activity before and after a diagnosis of diabetes is essential to preventing cardiovascular disease. Early intervention strategies are necessary to promote physical activity and exercise routines after a diagnosis of diabetes in people with obesity and those with pre-existing cardiovascular disease.