Background: Similar to programed death ligand 1 (PD-L1), indoleamine 2,3-Dioxygenase 1 (IDO1) is known to exert immunosuppressive effects and be variably expressed in human lung cancer. However, IDO1 expression has not been well-studied in lung adenocarcinoma (ADC). Methods: PD-L1 and IDO1 expression were evaluated in 261 resected ADC using tissue microarrays and H-scores (cutoff 5). We compared IDO1 with PD-L1 expression in association with clinical features, tumor-infiltrating lymphocytes (TILs), HLA class I (β-2 microglobulin; B2M) expression, molecular alterations, and patient outcomes. Results: There was expression of PD-L1 in 89 (34.1%) and IDO1 in 74 (28.5%) cases, with co-expression in 49 (18.8%). Both PD-L1 and IDO1 were significantly associated with smoking, aggressive pathologic features, and abundant CD8+ and T-bet+ (Th1 marker) TILs. PD-L1 expression and abundant CD8+ were inversely associated with a loss of B2M membranous expression (p = 0.0019 and p < 0.001, respectively). Compared to PD-L1+/IDO1+ and PD-L1+ only cases, significantly fewer IDO1+ only cases had abundant CD8+ and T-bet+ TILs (p < 0.001, respectively). PD-L1 expression was significantly associated with EGFR wild-type (p < 0.001) and KRAS mutants (p = 0.021), whereas there was no difference in IDO1 expression between different molecular alterations. As for survival, PD-L1 was significantly associated with decreased progression-free (PFS) and overall survival (OS), while IDO1 was associated only with decreased OS. Interestingly, there was a significant difference in the 5-year PFS and OS (p = 0.004 and 0.038, respectively), where cases without PD-L1 or IDO1 expression had the longest survival, and those with PD-L1 alone had the shortest survival. Conclusions: While PD-L1 +/- IDO1 expression is observed in association with B2M expression, CTL/Th1 microenvironments, EGFR wild-type, and KRAS mutations, isolated IDO1 expression does not demonstrate these associations, suggesting that IDO1 may serve a distinct immunosuppressive role in ADC. Thus, blockade of IDO1 may represent an alternative and/or complementary therapeutic strategy to reactivate anti-tumor immunity.