Abstract Background: GRAP2 is an adaptor protein involved in leukocyte-specific protein-tyrosine kinase signaling; however, the prognostic value of GRAP2 and its correlation with immune cell infiltration in lung adenocarcinoma (LUAD) remain unclear.Methods: All original data were downloaded from the TCGA database and integrated via R 3.2.2. GRAP2 expression was explored with the TCGA and TIMER databases. We evaluated the influence of GRAP2 on clinical prognosis using the Kaplan-Meier plotter, Gene Expression Omnibus (GEO) database and GEPIA database. Correlations between GRAP2 and cancer immune characteristics were analyzed via TIMER and TISIDB databases. Finally, we confirmed the expression of GRAP2 in LUAD by immunohistochemistry staining.Results: Transcription levels of GRAP2 were significantly lower in several human cancers, including LUAD, than in adjacent normal tissues. We also found that tumor tissues have lower protein expression levels of GRAP2 compared with adjacent normal tissues in LUAD by immunohistochemistry staining. The down-regulated GRAP2 was associated with poorer overall survival, pathologic stage, T stage, N stage and primary therapy outcome in LUAD. Mechanically, we identified a hub gene that included a total of 91 GRAP2 co-expressed genes, which were tightly associated with immune response in LUAD. GRAP2 expression was positively correlated with infiltrating levels of B cells, CD8+ T cells, dendritic cells, eosinophils, macrophages, mast cells, Th2 cells, Th1 cells, Th17 cells, NK cells and neutrophils. GRAP2 expression level also affected the cumulative survival time of B cells and dendritic cells. GRAP2 expression is positively correlated with multiple immune markers, chemokines, chemokine receptors and MHC molecules of LUAD.Conclusions: These findings suggest that GRAP2 is a tumor suppressor gene and can be used as a prognostic biomarker for determining prognosis and immune infiltration in LUAD.