Objectives <p>Inducible T cell costimulator (ICOS) is a CD28-family costimulatory receptor with bidirectional therapeutic effects on antitumor immunity, and HLA-mediated tumor recognition is essential for effective T cell responses. However, their clinicopathological significance in lung adenocarcinoma (LUAD) is not fully understood. We investigated subset-specific pattern of ICOS expression and examined whether ICOS⁺ tumor-infiltrating lymphocyte (TIL) density and tumor HLA expression provide immune context and prognostic information in LUAD.</p> Materials and Methods <p>We examined 228 resected LUADs by immunohistochemistry. ICOS⁺ TIL density and HLA class I and HLA-DR status were compared with the densities of CD8⁺, CD3⁺CD8⁻, and FOXP3⁺ TILs and with postsurgical outcomes. Subset-specific ICOS expression was evaluated using multiplex pseudocolored immunohistochemistry in 10 representative cases selected from the ICOS⁺ TIL-high group and validated using a publicly available single-cell RNA-seq dataset.</p> Results <p>ICOS⁺ TIL density correlated with each T cell subset. Multiplex analysis showed the highest ICOS positivity among Tregs, followed by CD4⁺ non-Tregs and CD8⁺ TILs. Because CD4⁺ non-Tregs were numerically predominant, ICOS⁺CD4⁺ non-Tregs constituted the largest ICOS⁺ fraction. Single-cell RNA-seq analysis corroborated these findings. ICOS⁺ TIL density was significantly higher in HLA-DR-strong tumors, and tumor HLA class I and HLA-DR were associated with longer recurrence-free survival. ICOS had no overall prognostic impact; however, among HLA-DR-strong tumors, ICOS-high patients tended toward longer cancer-specific survival.</p> Conclusion <p>In LUAD, ICOS⁺ TIL density largely reflects ICOS⁺CD4⁺ non-Treg infiltration and is enriched in HLA-DR-strong tumors. Tumor HLA expression was associated with favorable prognosis, and ICOS may have context-dependent clinical significance that warrants further investigation.</p>