Background: Adjuvant chemotherapy provides limited benefit in unselected stage II colon cancer. Post-operative circulating tumour DNA (ctDNA) has higher prognostic value than classical clinical markers, with ctDNA positivity indicating an unfavourable outcome. Patients and methods: Patients with Union for International Cancer Control stage II, mismatch repair proficient/microsatellite stable colon cancer were tested for ctDNA using an academic, tumour-informed, next-generation sequencing-based test. ctDNA-positive patients were randomly assigned 2:1 to CHEMO (capecitabine ± oxaliplatin) or observation (OBS). ctDNA-negative patients were randomly assigned 1:4 to OBS or OFF-STUDY. ctDNA results were not disclosed in the OBS group. The primary endpoint was disease-free survival (DFS) in ctDNA-positive patients. All differences were tested using one-sided log-rank tests. The trial ended early due to funding expiry. Results: From June 2020 to July 2025, 2126 patients were screened in Germany and Austria. Overall, 1396 patients (2.9% ctDNA-positive) were randomly assigned: 1083 to OFF-STUDY, 287 to OBS, and 26 to CHEMO, of whom 81% started therapy. DFS and overall survival (OS) were significantly worse in ctDNA-positive versus ctDNA-negative patients [3-year DFS 52% versus 87%, hazard ratio (HR) 4.28, 95% confidence interval (CI) 2.32-7.93, P < 0.001; 3-year OS 88% versus 98%, HR 5.48, 95% CI 1.64-18.28, P = 0.001]. In the intention-to-treat (ITT) cohort, the between-arm differences were not significant (3-year recurrence 36% versus 62%, HR 0.48, 95% CI 0.17-1.33, P = 0.075; 3-year DFS 61% versus 38%, HR 0.55, 95% CI 0.21-1.48, P = 0.12). In the per-protocol analysis (excluding untreated CHEMO patients), time to recurrence and DFS were improved with CHEMO compared with OBS (3-year recurrence 19% versus 62%, HR 0.23, 95% CI 0.06-0.87, P = 0.009; 3-year DFS 77% versus 38%, HR 0.31, 95% CI 0.09-1.03, P = 0.021). Conclusion: The primary ITT endpoint was not met, potentially related to the reduced power due to premature trial closure. The per-protocol analysis suggests a benefit from adjuvant therapy in ctDNA-positive patients, supporting ctDNA testing for adjuvant decision making in the future.