Janus kinase 1 (JAK1) is a member of the JAK family, which plays an essential and non-redundant role in tumorigenesis. However, the potential role of JAK1 in immune infiltration and prognosis of lung adenocarcinoma (LUAD) remains unclear. The mRNA expression and methylation level of JAK1 in LUAD were examined using the Oncomine and The Cancer Genome Atlas (TCGA) databases, respectively. The correlations between JAK1 expression and its methylation level and clinicopathological parameters were analyzed. The Kaplan–Meier plotter database was used to evaluate the prognostic value of JAK1 in LUAD. The signaling pathways associated with JAK1 expression were identified by performing a GSEA. The CIBERSORT and TIMER databases were used to analyze the correlations between JAK1 and tumor-infiltrating immune cells. In addition, the JAK1 expression and proportion of immune cells in LUAD cell lines were analyzed. The JAK1 expression was remarkably decreased in patients with LUAD and significantly correlated with the clinical features of patients with LUAD. The JAK1 methylation level was increased and negatively correlated with its mRNA expression. A decrease in JAK1 expression was correlated with poor prognosis. The results of GSEA showed that cell adhesion, tumorigenesis, and immune-related signaling pathways were mainly enriched. JAK1 was positively associated with tumor-infiltrating immune cells, and the results of CIBERSORT analysis suggested that JAK1 was correlated with monotypes and M1 macrophages. The results of the TIMER database analysis confirmed that JAK1 was closely associated with the gene markers of M1 macrophages. Thus, JAK1 may serve as a potential prognostic biomarker in LUAD and is associated with immune infiltration.