Background: Chemoradiotherapy (CRT) followed by durvalumab as maintenance therapy prolonged progression-free survival (PFS) and overall survival (OS) in unresectable locally advanced NSCLC (LA-NSCLC). Additionally, the history of radiotherapy and CRT has been reported to increase the efficacy of PD-1 blockade in advanced NSCLC patients. We evaluated the efficacy of anti-PD-(L)1 antibody therapy after CRT failure, and how CRT changes the status of PD-L1 expression on tumors and tumor infiltrated lymphocytes (TILs) in tumor microenvironment (TME) in unresectable LA-NSCLC patients. Methods: We retrospectively reviewed unresectable LA-NSCLC patients treated with CRT between December 2007 and December 2018, and evaluated the efficacy of PD-1 blockade after CRT failure. We also analyzed PD-L1 (clone: 22C3) expression on tumor cells, and CD8 positive TILs using the paired specimens that had been obtained pre-CRT and post-CRT failure. Results: We identified 422 patients who received CRT. Median follow-up was 36.1 months (range 2.7–138.1 months). Among these patients, sixty-five patients who had progressed post-CRT received anti-PD- (L)1 therapy (PD-1 therapy: 61 patients, PD-L1 therapy: 4 patients). Response rate (RR) and PFS of anti-PD-(L)1 therapy were 48% (95% CI, 35–60) and 8.7 (95% CI, 4.5–13.0) months. The RR and PFS did not differ according to PD-L1 expression levels (Table). Of the 18 patients, 9, 7, and 2 showed upregulation in PD-L1 expression or down- or no change, respectively, post-CRT. In contrast, the density of CD8 positive TILs in TME increased by CRT treatment ([pre-CRT]: median, 110 ± 239 /mm² vs. [post-CRT]: median, 470 ± 533 /mm², p = 0.025). Conclusions: The clinical outcome of anti-PD-(L)1 therapy after CRT failure in LA-NSCLC patients could be better than advanced NSCLC patients, but did not differ according to PD-L1 expression levels. The efficacy of PD-(L)1 therapy enhanced by CRT treatment could be due to the infiltration of CD8 T-cells into TME.