Background. As a marker of differentiation, Killer cell lectin like receptor G1 (KLRG1) plays an inhibitory role in human NK cells and T cells. However, its clinical role remains inexplicit. This work intended to investigate the predictive ability of KLRG1 in lung adenocarcinoma (LUAD) after immune-checkpoint inhibitor therapy, as well as to explore the role of a possible KLRG1 molecular mechanism on LUAD development.Methods. Using data from the Gene Expression Omnibus, the Cancer Genome Atlas and the Genotype-Tissue Expression, we compared the expression of KLRG1 and its related genes Bruton tyrosine kinase (BTK), C-C motif chemokine receptor 2 (CCR2), Scm polycomb group protein like 4 (SCML4) in LUAD and normal lung tissues. We further established a stable LUAD cell line with KLRG1 knockdown and investigate the effect of KLRG1 knockdown on tumor cell proliferation. We also studied the prognostic value of the four factors in terms of overall survival (OS) in LUAD. Using data from the Gene Expression Omnibus, we further investigated the expression of KLRG1 in the patients with different responses after immunotherapy.Results. The expression of KLRG1, BTK, CCR2 and SCML4 was significantly downregulated in LUAD tissues compared to normal controls. Knockdown of KLRG1 promoted the proliferation of A549 tumor cells. And low expression of these four factors was all associated with unfavorable overall survival in patients with LUAD. Furthermore, low expression of KLRG1 also correlated with poor responses in LUAD patients after immunotherapy.Conclusion. Based on these findings, we infer that KLRG1 had significant correlation with immunotherapy response. Meanwhile, KLRG1, BTK, CCR2 and SCML4 might serve as valuable prognostic biomarkers in LUAD. Key pointsKLRG1 inhibits the progress of LUAD.KLRG1 had significant correlation with immunotherapy response.KLRG1, BTK, CCR2 and SCML4 might serve as valuable prognostic biomarkers in LUAD.