Background: In the IDEA collaboration, noninferiority was not confirmed for 3 months versus 6 months of FOLFOX adjuvant chemotherapy among patients with high-risk stage II and stage III colon cancer (CC). Patients with deficient DNA mismatch repair (dMMR) have a good prognosis, but for whom, whether limiting the duration of adjuvant therapy will compromise oncologic outcomes remains undefined. We evaluated the impact of 3 months of FOLFOX adjuvant chemotherapy or surgery alone in comparison with 6 months of FOLFOX on disease-free survival (DFS) in dMMR CC patients. Methods: This retrospective study included all consecutive patients who underwent curative surgical resection for high-risk stage II or III dMMR CC between May, 2011 and July, 2019. Prognostic factors were analyzed using Cox models, and hazard ratios (HRs) with 95% confidence intervals (CIs) were calculated. Results: A total of 242 dMMR CC patients were included (43.4% high-risk stage II, 56.6% stage III). The patients received 6 months of FOLFOX adjuvant chemotherapy (n = 66; median cycles [rang] = 12 [10-12]), 3 months of FOLFOX (n = 87; median cycles [rang] = 6 [4-8]), or surgery alone (n = 89). Three groups were generally well balanced, although more patients with stage III were in the 6-month therapy group (74.2%), compared with the 3-month therapy group (57.5%) and the surgery alone group (42.7%). As compared with 6 months of FOLFOX adjuvant chemotherapy in the overall population, 3 months therapy reduced DFS in multivariable analysis (HR, 2.78; 95CI, 1.18 to 6.47; P = 0.02), similar to surgery alone (HR, 2.30; 95CI, 0.99 to 5.38; P = 0.05). In the subgroup analysis, a therapy duration of 6 months was statistically superior to a duration of 3 months only in the patients with stage III, with a 3-year rate of DFS of 86.2% versus 70.8% (HR, 3.06; 95% CI, 1.14 to 8.19; P = 0.026). Conclusions: This study supports the 6-month duration of FOLFOX adjuvant chemotherapy in stage III dMMR CC.