Background: In stage III colon cancer patients treated with CAPOX, 3 months of therapy was as effective as 6 months. It was not the case for those receiving mFOLFOX6. We assessed the prognostic and predictive value of the Immunoscore (IS) in the mFOLFOX6 subgroup (90% of patients enrolled) of the IDEA France cohort study. Methods: 1200 patients randomly assigned to 3 months (n = 593) or 6 months (n = 607) of mFOLFOX6, with available tumor sample, were included. Densities of CD3+ and cytotoxic CD8+ T-cells in the tumor and invasive margin were determined by immunohistochemistry, quantified by digital pathology, and converted to IS using the pre-defined cut-off. The IS performance to predict disease-free survival (DFS) was assessed in each arm and adjusted in multivariate Cox models. Results: In a two-category IS analysis, Low and (Int+High) IS were observed in 423 (43.5%) and 550 (56.5%) patients, respectively. Low IS identified patients with higher-risk of relapse or death (HR = 1.60; 95% CI 1.27-2.01, P < 0.0001). The 3-year DFS was 66.34% (95% CI 61.54-70.69) and 77.66% (95% CI 73.86-80.97) for Low and (Int+High) IS, respectively. In multivariate analysis, IS remained independently associated with DFS (P = 0.0007) when combined with T/N stage. A statistically significant interaction was observed for the IS predictive value for treatment duration (3 vs 6 months) in term of DFS in the whole population (P = 0.0566) and TN subgroups (Low-risk T1-3, N1 vs High-risk T4 and/or N2; P = 0.0015). The 3-year DFS of patients with (Int+High) IS in the 3-month arm was 71.5% (95% CI 65.7-76.6) versus 83.8% (95% CI, 78.8-87.8) in the 6-month arm (HR = 0.528; 95% CI 0.372-0.750; log-rank P = 0.0004); the benefit retained in low-risk and high-risk patients (all P≤0.010). 6-month mFOLFOX6 showed no significant benefit for patients with Low IS (HR = 0.836, log-rank P = 0.269). Conclusions: The IS prognostic value in patients treated with mFOLFOX6 was confirmed. Its predictive value for benefit of longer duration treatment, despite a strong statistical signal, needs to be confirmed in an external validation cohort. Clinical trial information: NCT03422601