Adjuvant chemotherapy in stage III colon cancer provides uncertain benefit at the individual level. Circulating tumor DNA (ctDNA) may help refine risk-adjusted treatment selection. In this multicenter, randomized, phase 2/3 trial, patients with stage III colon cancer underwent ctDNA testing 5-6 weeks after surgery and were assigned (1:1) to ctDNA-guided or standard management. In the ctDNA-guided arm, patients negative for ctDNA received de-escalated therapy, whereas ctDNA-positive patients received escalated therapy. Clinicians prespecified the standard regimen. Primary endpoints were 3-year recurrence-free survival (RFS) for ctDNA-negative patients and 2-year RFS for ctDNA-positive patients. Secondary endpoints included treatment-related hospitalization and ctDNA clearance. Among 968 evaluable patients, 702 (72.5%) were ctDNA negative. With a median follow-up of 47 months, ctDNA-negative patients experienced significantly fewer recurrences than ctDNA-positive patients (3-year RFS 87% versus 49%; P < 0.001). In ctDNA-negative patients, de-escalation reduced oxaliplatin use (34.8% versus 88.6%) and hospitalizations (8.5% versus 13.2%) but yielded slightly lower RFS than standard management (85.3% versus 88.1%), not meeting the non-inferiority margin. In ctDNA-positive patients, higher ctDNA burden correlated with recurrence risk (3-year RFS 77% to 23% across quartiles; P < 0.001). Escalated therapy did not improve outcomes over standard management (2-year RFS 51% versus 61%). There was no unexpected toxicity. Persistent ctDNA after treatment predicted markedly worse prognosis (3-year RFS 14% versus 79%). ctDNA is validated as a strong prognostic classifier. ctDNA-guided de-escalation reduced oxaliplatin exposure and adverse events with outcomes approaching standard of care, whereas exploratory chemotherapy intensification conferred no RFS benefit, suggesting a need for novel strategies in ctDNA-positive disease.Australian New Zealand Clinical Trials Registry Identifier: ACTRN12617001566325 .