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Breast cancer is the leading cause of cancer death among Brazilian women, yet mortality estimates are often underestimated due to ill-defined causes, incomplete diagnoses, underreporting, and data quality limitations. Using national mortality data from 2000 to 2023, we examined the spatial distribution of breast cancer mortality among women aged 20 years and older, comparing uncorrected and adjusted estimates. Adjustments were applied to correct ill-defined causes, incomplete diagnoses, underreporting, and other data quality limitations using methods developed by the World Health Organization and the Brazilian Institute of Geography and Statistics (IBGE). Age-standardized mortality rates were calculated for five time periods using the World Health Organization (WHO) standard population, and spatial patterns were analyzed using choropleth maps, Moran I, and Local Indicators of Spatial Association (LISA). A total of 328,319 breast cancer deaths were reported, increasing to 385,068 (+ 17.3%) after adjustment. Corrections had the greatest effect in the North and Northeast in 2000-2004 (up to + 69.9%), but declined substantially over time. Mortality remained consistently higher in wealthier regions, while adjustments revealed underestimation in historically underserved areas. These findings reveal enduring geographic inequalities in breast cancer mortality and underscore the urgent need for targeted interventions and improved surveillance systems.