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Objectives Age-standardised incidence rates of myocarditis have been widely reported; this metric primarily provides a cross-sectional view of risk based on a fixed standard population and does not capture the effects of population ageing. Lifetime risk represents the probability of developing myocarditis over an individual’s lifespan and incorporates both demographic ageing and competing mortality, thereby offering a complementary perspective that reflects the burden experienced in real-world populations. This study aimed to estimate the global lifetime risk of myocarditis.Design This was a systematic analysis using publicly available aggregated data from Global Burden of Disease Study 2021.Setting The study included 204 countries and territories from 1990 to 2021.Participants We used deidentified data from the general population to measure the lifetime risk of developing myocarditis.Primary and secondary outcome measures The primary outcome was the estimated lifetime risk of developing myocarditis at the global, regional and national levels. Secondary outcomes included long-term trends in lifetime risk using the average annual percentage change (AAPC), variation in lifetime risk by age and sex, differences across Sociodemographic Index (SDI) levels assessed with concentration indices, and future risk estimated using an Autoregressive Integrated Moving Average (ARIMA) model.Results The global lifetime risk of developing myocarditis increased from 1.33% (95% CI 1.32% to 1.33%) in 1990 to 1.50% (95% CI 1.49% to 1.50%) in 2021 (AAPC: 0.388%, 95% CI 0.314% to 0.462%). High and high-middle SDI countries had the greatest risks. In 2021, risk was higher in males (1.59%, 95% CI 1.59% to 1.60%) than in females (1.40%, 95% CI 1.39% to 1.40%). People over 40 years, especially older women, had higher lifetime risks. Forecasts based on the ARIMA model suggest that the risk will continue to rise through 2050.Conclusions From 1990 to 2021, the global lifetime risk of developing myocarditis has shown an upward trend. It is crucial to enhance early detection and diagnostic capabilities in primary care settings.