MicroRNAs (miRNAs) play an important role in the regulation of human cancers, including breast cancer (BC). In the current study, we examined the expression pattern of the miRNA miR‐125a‐5p in human BC tissues, tumorigenesis of BC progression. We found that miR‐125a‐5p was significantly downregulated in human BC tissues. Overexpression of miR‐125a‐5p in a xenograft mouse model indicated that miR‐125a‐5p may function as a tumour suppressor during carcinogenesis. To explore the molecular mechanism by which miR‐125a‐5p contributes to BC progression, we predicted the target genes of miR‐125a‐5p and identified BC susceptibility gene 1–associated protein 1 (BAP1) as a direct target. Finally, we demonstrated that BAP1 had opposing effects to those of miR‐125a‐5p on BC cells, suggesting that miR‐125a‐5p may inhibit cell proliferation and promote cell apoptosis by negatively regulating BAP1. Taken together, our findings provide the first clues regarding the role of miR‐125a‐5p as a tumour suppressor in BC via the inhibition of BAP1 translation. Our research indicated that miR‐125a‐5p can negatively regulate breast cancer susceptibility gene 1–associated protein 1 (BAP1), resulting in the inhibition of breast cancer (BC) tumorigenesis in vivo and in vitro. This suggests that a novel strategy for BC treatment may be found after further research on miR‐125a‐5p and BAP1.