Background: Postoperative morbidity can reduce quality of life, physical performance, and tolerability of postoperative chemotherapy in patients with colorectal liver metastases (CRLM). Exercise can improve these outcomes in some cancer populations. However, it remains unknown whether exercise can be delivered in the early postoperative period following surgery for CRLM without increasing the risk of harms. Objective: The primary objective was to compare the number of serious adverse events (SAEs) with exercise intervention versus control. The secondary objectives were to compare non-SAEs, chemotherapy dose modifications, patient-reported outcomes, cardiorespiratory fitness, and physical performance. Methods: Patients with CRLM scheduled to open surgery with or without postoperative chemotherapy were randomized 2:1 to intervention or control. The intervention group performed 30-50 min low-to-high intensity exercise 5 times/week for 8 weeks, initiated one day after postoperative hospital discharge. The primary outcome was SAEs. The secondary outcomes were SAEs; chemotherapy dose modifications; patient-reported outcomes; cardiorespiratory fitness; and physical performance. Results: Fifty-five participants were randomized. The number of SAEs was similar between the groups (between-group difference [95% CI]: -0.07 [-0.59; 0.43] events), whereas the number of non-SAEs was lower in intervention (between-group difference [95% CI]: -4.65 [-9.14; -0.17] events). We found between-group differences in time to postoperative chemotherapy (intervention: 25 days, control; 42 days) and chemotherapy dose modifications (RR [95%CI]: 0.55 [0.35; 0.88]). Additionally, we found between-group differences in quality of life, cardiorespiratory fitness, and physical performance, in favor of intervention. Conclusions: Early-onset postoperative exercise exhibit a favorable harms-benefit profile in patients with CRLM. This warrants further investigation in larger randomized controlled trials. Trial registration: Prospectively registered at clinicaltrials.gov (NCT04751773).