Background:　Understanding　trends　in　atrial　fibrillation　(AF)　and　atrial　flutter　(AFL)　is　crucial　for　effective　prevention.　This　study　quantifies　the　burden　and　identifies　key　risk　factors　for　AF/AFL　across　204　countries　and　territories　from　1990　to　2021.　Methods:　Using　data　from　the　Global　Burden　of　Disease　(GBD)　study　2021,　we　employed　average　annual　percentage　change　(AAPC)　and　Bayesian　Age　Period　Cohort　(BAPC)　modelling　to　assess　trends　and　future　projections,　with　a　focus　on　disparities　across　SocioDemographic　Index　(SDI)　levels.　Results:　In　2021,　AF/AFL　remained　the　most　common　arrhythmia　globally,　with　a　prevalence　of　52.55　million,　8.36　million　disability-　adjusted　life　years　(DALYs),　and　0.34　million　deaths.　For　each　0.1　increase　in　SDI,　age-standardized　mortality　rates　increased　by　4.94%,　DALYs　by　2.56%,　and　prevalence　rates　by　2.40%,　highlighting　the　significant　impact　of　socioeconomic　development　on　AF/AFL　burden.　AAPC　analysis　indicated　slight　increases　in　deaths,　prevalence,　and　DALYs,　along　with　a　decline　in　incidence,　reflecting　the　impact　of　public　health　strategies.　However,　significant　inequalities　were　observed　across　SDI　levels,　with　a　higher　burden　in　high-SDI　regions　because　of　aging　populations　and　improved　detection,　whereas　lower　SDI　regions　faced　higher　age-standardized　mortality　rates　because　of　resource　constraints.　Conclusions:　By　2035,　we　project　a　4.07%　increase　in　incidence　despite　a　6.84%　decrease　in　mortality.　Nearly　one-half　of　the　AF/AFL　burden　is　linked　to　modifiable　risk　factors,　underscoring　the　importance　of　tailored　prevention　strategies　and　improved　health　care　access,　especially　in　lower　SDI　countries,　to　mitigate　rising　disease　rates　and　reduce　health　care　disparities.