The FDA approved atezolizumab plus chemotherapy for adjuvant treatment of stage III dMMR colon cancer after the ATOMIC phase III trial reduced the risk of recurrence or death by 50%. The decision adds to the growing use of immune checkpoint inhibitors in earlier-stage colorectal disease when mismatch repair deficiency defines the biology. Clinicians will now have a new post-surgery option for this subset, where conventional adjuvant chemotherapy historically has been the standard path. Trial endpoints focused on recurrence-free survival and overall clinical outcomes consistent with adjuvant benefit. For developers, the approval strengthens the clinical and regulatory signal that biomarker-defined dMMR biology can support immunotherapy intensification outside the metastatic setting. It also increases the competitive pressure on other perioperative checkpoint programs in biomarker-enriched colorectal cohorts.