Background: Genomic characterization of breast cancer brain metastases (BCBMs) has thus far been limited. The objective of this study was to describe the landscape of genomic alterations in patients (pts) with BCBMs. Methods: Targeted next-generation DNA sequencing of > 300 cancer-related genes (OncoPanel) was prospectively performed on primary and metastatic (met) tumors in 321 pts with a diagnosis of BCBM between August 2016 and April 2019 at Dana-Farber Cancer Institute (table). Enrichment analysis of genomic alterations was performed using a two-sided Fisher exact test and differences in tumor mutation burden (TMB) between groups were assessed using two-sided Mann-Whitney U test. Multiple comparison correction was performed using the Benjamini-Hochberg procedure. Results: All subtypes were represented in BCBM (25 HR+/HER2-; 24 HR+/HER2+; 27 HR-/HER2+; 18 TNBC; 5 unknown; n = 99) and extracranial (EC) samples: (96 HR+/HER2-; 32 HR+/HER2+; 22 HR-/HER2+; 41 TNBC; 31 unknown; n = 222). BCBMs were found most commonly to have mutations or copy number alterations in TP53, ERBB2, PIK3CA, GATA3, PTEN, ESR1, CDH1, BRCA2, ARID1A, BRCA1 (>5% frequency, table). Two pts acquired ERBB2 amplification (amp) between the matched primary breast sample and brain met. In pair-wise comparisons of BCBMs to unmatched primaries or EC mets, only ERBB2 amp was significantly enriched (table, † = adjusted p < 0.05). There was no significant difference in TMB between BCBM and EC mets (median 9.12 vs 7.26, p = 0.15). In contrast, TMB was significantly higher in BCBMs compared to unmatched primaries (median 9.12 vs 7.26, p=0.005). Conclusions: BCBMs display similar mutations and copy number alterations compared to primary tumors and EC mets in pts with BCBM. These data suggest that BCBMs contain actionable genomic alterations that are most often also reflected in EC disease. Alterations in ERBB2, PIK3CA/PTEN, and BRCA1/2 represent potentially targetable alterations in pts with BCBM.