Breast cancer is the most prevalent malignant disease among women and affects women of reproductive age (15-49 years) across the globe. This study examines patterns and trends in the epidemiology of breast cancer in women of reproductive age using global burden of disease data (1990-2021), with the objective of informing targeted public health strategies. Using data from the Global Burden of Disease Study (GBD 2021), this study analyzed the temporal trends, regional disparities, and health inequalities in the burden of breast cancer among women of reproductive age across 204 countries and territories from 1990 to 2021. The analysis employed methodologies including the age-period-cohort (APC) model, decomposition analysis, the slope index of inequality (SII), and the concentration index (CIX). Additionally, the Bayesian age-period-cohort (BAPC) predictive model was utilized to forecast the global burden of breast cancer among women of reproductive age from 2022 to 2040. From 1990 to 2021, the global burden of breast cancer among women of reproductive age showed a significant upward trend, with increases of 118.7%, 121.3%, and 66.8% in the number of new cases, prevalence, and disability-adjusted life years (DALYs) lost, respectively. The most pronounced increases were observed in regions with low and low-middle sociodemographic index (SDI). APC analysis revealed that the rising incidence and prevalence rates were driven by age and period effects, while the risk of breast cancer declined among more recent birth cohorts. Decomposition analysis indicated that population aging and growth were the primary contributors to the global rise in breast cancer burden. SII and CIX analyses demonstrated that absolute health inequalities in breast cancer burden intensified during this period, while relative inequalities showed a slight decline. According to the BAPC predictive model, by 2040, the number of new breast cancer cases among women of reproductive age is expected to increase by 47.8% compared to 2022, accompanied by continued rises in age-standardized incidence, prevalence, and mortality rates. Between 1990 and 2021, the global burden of breast cancer among women of reproductive age increased significantly, with notable health inequalities across regions and populations. By 2040, new cases are projected to rise by 47.8% from 2022, underscoring an urgent global challenge in breast cancer prevention and control.