Background: In VOLTAGE-A, chemoradiotherapy (CRT; 50.4Gy with capecitabine, 1,650mg/m²) followed by five cycles of consolidation nivolumab (nivo) (240mg q2 weeks) showed 30% pathological complete response [pCR; AJCC tumor regression grade (TRG) 0] and 38% major pathological responses (AJCC TRG 0-1) in patients with microsatellite stable (MSS) locally advanced primary rectal cancer (LARC). In addition, 60% pCR was observed in patients with microsatellite instability-high (MSI-H) LARC. In this study, we aimed to determine the predictive biomarkers for efficacy of sequential preoperative CRT and consolidation nivo. Methods: Serial tumor biopsies were performed at four time points:pre-CRT; post-CRT; post-3 cycles of nivo; and pre-surgery. We analyzed the immune status of the patients by flow cytometry using the collected tumor-infiltrating lymphocytes (TILs) dissociated from tumor samples. Whole exome and RNA sequencing analyses were conducted using the extracted DNA and RNA from tumor, respectively. The PD-L1 status of tumor samples was also evaluated by in vitro diagnostic immunohistochemistry staining. Results: Of the 38 MSS patients whose PD-L1 tumor proportion score (TPS) was analyzable in pre-CRT samples, the pCR rates were 67% (6/9) and 17% (5/29) in positive (≥1%) and negative PD-L1 status (p = 0.009), respectively. Among the 24 MSS patients whose samples were serially collected, the pCR rates according to CD8⁺ T cells/effector regulatory T cells (CD8/eTreg) ratio in TILs of pre-CRT samples ≥2.5 and < 2.5 were 78% (7/9) and 13% (2/15), respectively (p = 0.003). The CD8/eTreg ratio in TILs was consistently high in patients with pCR during the study treatments. Ki-67 and PD-1 expression by CD8⁺ T cells in TILs was significantly high in pre-CRT samples from patients TRG 0-1. Conversely, in patients with TRG 2-3, CTLA-4 expression by both CD4⁺ T cells and CD8⁺ T cells in TILs was significantly high after five cycles of nivo, suggesting the potential resistance mechanisms of nivo monotherapy. Of the 21 MSS patients whose consensus molecular subtype (CMS) was analyzable, those with CMS1 and CMS3 tumors achieved 100% (2/2) and 60% (4/6) TRG 0-1, respectively. In contrast, patients with CMS2 and CMS4 tumors achieved 43% (3/7) and 29% (2/7) TRG 0-1, respectively. The tumor mutational burden (TMB) of pre-CRT samples in five MSI-H patients was higher than that in the 24 MSS patients (median: 13.2 vs. 0.99 mutation/Mbp, p < 0.0001). Among MSS patients, TMB was higher in patients with TRG 0-1 than in patients with TRG 2-3 (median: 1.45 vs. 0.84 mutation/Mbp, p = 0.016). Conclusions: Positive PD-L1 TPS; high CD8/eTreg ratio; Ki-67, PD-1, and CTLA-4 expression by CD8⁺ T cells in TILs; CMS 1 or 3; and high TMB can be good predictors of efficacy of preoperative CRT followed by nivo. Clinical trial information: NCT02948348