ObjectiveUsing the GBD 2021 database, this study assessed the global burden and temporal trends of early onset breast cancer (EOBC) and projected future trajectories to 2046. MethodsData on EOBC incidence, mortality, and DALYs were extracted from the GBD 2021 study, covering 204 countries and territories. Trends were analyzed using joinpoint regression, APC models, inequality indices and Nordpred method. ResultsIn 2021, EOBC accounted for 567 903 incident cases and 131 024 deaths globally, contributing to 6 743 070 DALYs. The ASIR reached 24.56 per 100 000 population, with corresponding ASMR and ASDR of 7.9 and 239.03 per 100 000, respectively. Temporal trends revealed a modest global increase in ASIR (EAPC: +0.38%), contrasted by declines in ASMR and ASDR (EAPC: ?0.59% and ?0.46%, respectively). The APC model showed that the relative risk of EOBC increased with age and period effects and decreased with cohort effects. Although differences in EOBC burden between different SDI quintiles have narrowed slightly, significant inequities remain, with low SDI areas disproportionately affected. Projections for the year 2046 indicate that the burden of disease will continue to trend upwards. ConclusionEOBC burden has risen significantly, with marked inequalities across regions and SDI levels, which underscores the need for targeted early screening and equitable healthcare strategies. Global efforts must prioritize preventive measures, access to diagnostics, and tailored therapies to mitigate the projected increase in EOBC cases and reduce health inequities.