In phase I, we investigated the feasibility of sequentially-combined CRT, 5 cycles of nivolumab, and radical surgery. In phase II, patients with microsatellite stable (MSS) and microsatellite instability-high (MSI-H) LARC were evaluated. A pathological complete response (pCR) was centrally confirmed in 30% (11/37; 90% CI, 18 %-44%) and 60% (3/5) of the MSS and exploratory MSI-H cohorts, respectively. PD-L1 expression and an elevated CD8/eTreg ratio were positive predictors in patients with MSS LARC.