Background: The Immunoscore (IS), which has been shown to prognostically classify Stage I-III colon cancer (CC) patients, was assessed in the IDEA France cohort study evaluating 3 versus 6 months of oxaliplatin-based adjuvant chemotherapy in Stage III CC patients. Methods: Densities of CD3+ and cytotoxic CD8+ T cells in the tumor and invasive margin of each patient were quantified by digital pathology and converted to IS using pre-defined published cut-off. The performance of IS to predict disease-free survival (DFS) was assessed in the modified intention-to-treat population, in each study arm, and was adjusted with relevant clinical features in multivariable Cox models. Harrell’s C-statistics was used to investigate the IS performance. Results: 1322 patients were included; 82 were excluded due to pre-analytical non-conformity. IS was successfully analyzed in 1062 (85.6%) eligible patients. In a 2-category IS analysis, Low and (Int+High) IS were observed in n=599 (43.6%) and n=463 (56.4%) patients, respectively. IS was significantly correlated with T stage, T/N stage (T1-3 and N1 versus T4 and/or N2), and microsatellite instability status. The study met its primary objective of validating that Low IS identifies patients with higher-risk of relapse or death [HR=1.54; 95%CI 1.24-1.93, p=0.0001]. The 3-year DFS rates were 66.80% [95%CI 62.23-70.95] and 77.14% [95%CI 73.50-80.35] for Low IS and (Int+High) IS, respectively. In multivariable analysis, IS remained independently associated with DFS (p<0.0012) when combined with T/N stage. The addition of IS to the T/N stage significantly improved the model discrimination capacity [bootstrap C index mean difference, 0.022; 95%CI 0.005-0.04]. In addition, IS in 3 categories (Low, Int, High) and as a continuous variable were also both significantly associated with DFS (all p<0.001). In univariable analysis, IS was also associated with DFS in 6 months arm (p<0.0001); a similar trend was observed in 3 months arm (p=0.09). Conclusions: IS was confirmed as a prognostic factor of DFS in Stage III CC patients in the prospective IDEA France cohort study. Clinical trial information: NCT03422601