BACKGROUND: The number of frail patients with colorectal cancer (CRC) has increased. Despite evidence-based treatment guidelines, a large proportion of patients with resected CRC do not receive adjuvant chemotherapy in daily practice. This retrospective study aimed to examine the effect of adjuvant chemotherapy for CRC according to frailty. METHODS: We retrospectively analyzed data from 507 consecutive patients with curatively resected high-risk stage II or stage III CRC between 2009 and 2016. Frailty was assessed using the Clinical Frailty Scale (CFS): 1 (very fit) to 9 (terminally ill), and frailty was defined as CFS ≥ 4. Recurrence-free survival (RFS) and overall survival (OS) were compared between surgery alone and adjuvant chemotherapy in frail and non-frail patients. A cox proportional hazards model was used to calculate hazard ratios (HRs), controlling for potential confounders. RESULTS: Of the 507 patients, 194 (38%) were frail. There were no significant interactions between frailty and adjuvant chemotherapy regarding RFS (P = 0.59) and OS (P = 0.81). In multivariable analyses, associations of adjuvant chemotherapy with longer RFS and OS in frail patients (RFS, HR: 0.33, 95% CI 0.15-0.63; OS, HR: 0.23, 95% CI 0.08-0.54) were comparable to non-frail patients (RFS, HR: 0.36, 95% CI 0.22-0.58; OS, HR: 0.34, 95% CI 0.15-0.69). Frail patients receiving adjuvant chemotherapy were younger and had better nutritional status than those undergoing surgery alone (all P < 0.005). CONCLUSION: Selected frail patients with CRC may experience a similar survival benefit from adjuvant chemotherapy as non-frail patients. Clinical trials are needed to establish adjuvant chemotherapy for CRC in frail patients.