Objective To investigate the expression and clinical significance of multiple endocrine neoplasia type 1 (MEN1) gene in breast cancer. Methods The Cancer Genome Atlas (TCGA) database was used to analyze the relationship between MEN1 gene and clinicopathological characteristics of breast cancer. Kaplan-Meier method was used to observe the effect of MEN1 on survival of breast cancer patients. Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG), and gene set enrichment analysis (GSEA) were used to predict the function and signaling pathways of MEN1 related and interacting genes. Tumor Immune Estimation Resource (TIMER) and single sample gene set enrichment analysis (ssGSEA) were used to investigate the correlation between the level of immune infiltration and MEN1 expression in breast cancer. Results MEN1 was highly expressed in breast cancer patients, and its expression level was related to PAM50 subtype and menopause status (both P＜0.05). Kaplan-Meier survival analysis showed that high MEN1 expression was associated with poor clinical outcome (P＝0.019). GO and KEGG enrichment analysis showed that MEN1 related and interacting genes were involved in biological processes (histone modification, histone-lysine methylation), cell components (methyltransferase complex and histone methyltransferase complex), molecular functions (histone-methyltransferase activity), and functional pathways (transcriptional disorders in tumors). GSEA identified that the highly expressed MEN1 phenotype was involved in vesicle-mediated transport, complement cascade, B-cell receptor signaling, lymphocyte-non-lymphocyte interaction, interleukin signaling, and cytokine signaling pathways in the immune system. CancerSEA single cell sequencing results indicated that the expression of MEN1 was positively correlated with angiogenesis in MDA-MB-231 cells (P＜0.05). TIMER analysis found that MEN1 in breast cancer was negatively correlated with the infiltration levels of macrophages and CD8⁺ T cells, and positively correlated with the infiltration level of CD4⁺ T cells (all P＜0.05). ssGSEA showed that the infiltration levels of 18 types of immune cells were negatively correlated with MEN1 expression (all P＜0.05). Conclusion High MEN1 level is associated with poor survival and immune infiltration in breast cancer patients.