Clinical trials testing Janus kinase-1 (JAK1) inhibitors in cancers are under way. Whether the JAK1 mRNA levels in breast tumors correlates with outcome has not been evaluated. JAK1 expression was analyzed via the Oncomine database and Tumor IMmune Estimation Resource site. Tumor tissues from 57 breast cancer patients were used for qRT-PCR and immune infiltration assessment. JAK1 expression was significantly lower in breast invasive carcinoma compared with adjacent normal tissues. Public databases (Kaplan-Meier plotter and PrognoScan) showed that low JAK1 expression was associated with poorer survival. Data from The Cancer Genome Atlas (TCGA) showed that high JAK1 expression was associated with increased survival in both TNM I-II and TNM III-IV patients. JAK1 was inversely correlated with tumor size status, lymph node status, and TNM of breast cancer patients. JAK1 levels were correlated with the T cell transcript-enriched LYM metagene signature and was significantly lower in the low tumor infiltrating lymphocytes (TILs) group. JAK1 expression levels had significant positive correlations with infiltrating levels of CD8+ T cells, CD4+ T cells, macrophages, neutrophils, and dendritic cells in breast cancer and not with other B cells. In conclusion, JAK1 mRNA levels were correlated with prognosis and immune infiltrating levels in breast cancer.