Lung adenocarcinoma (LUAD) is a major pathological type of lung cancer. Understanding the mechanism of LUAD at the molecular level is important for a clinical decision. In this study, we use bioinformatic analysis to explore the prognostic value of P4HA1 in lung adenocarcinoma (LUAD) and the relationship with prognosis and tumor-infiltrating immune cells (TIICs). The results showed that the expression of P4HA1 was significantly higher in tumor tissues than in normal tissues for LUAD patients. Upregulated P4HA1 was related to stage and T classification. Kaplan-Meier analysis indicated that upregulation of P4HA1 was significantly related to worse overall survival (OS). Univariate and multivariate Cox analysis indicated P4HA1 remained to be an independent prognostic factor. GSEA showed that several cancer-related and immune-related signaling pathways exhibited prominently differential enrichment in P4HA1-high expression phenotype. In addition, the expression of P4HA1 was significantly correlated with proportion of several TIICs, particularly B cells and CD4+ T cells. In conclusion, our study confirmed that P4HA1 is a promising biomarker of poor prognosis and relates to immune infiltrates in LUAD.