IMPORTANCE: Unstable chest wall injuries have high rates of mortality and morbidity. In the last decade, multiple studies have reported improved outcomes with operative compared with nonoperative treatment. However, to date, an adequately powered, randomized clinical trial to support operative treatment has been lacking. OBJECTIVE: To compare outcomes of surgical treatment of acute unstable chest wall injuries with nonsurgical management. DESIGN, SETTING, AND PARTICIPANTS: This was a multicenter, prospective, randomized clinical trial conducted from October 10, 2011, to October 2, 2019, across 15 sites in Canada and the US. Inclusion criteria were patients between the ages of 16 to 85 years with displaced rib fractures with a flail chest or non-flail chest injuries with severe chest wall deformity. Exclusion criteria included patients with significant other injuries that would otherwise require prolonged mechanical ventilation, those medically unfit for surgery, or those who were randomly assigned to study groups after 72 hours of injury. Data were analyzed from March 20, 2019, to March 5, 2021. INTERVENTIONS: Patients were randomized 1:1 to receive operative treatment with plate and screws or nonoperative treatment. MAIN OUTCOMES AND MEASURES: The primary outcome was ventilator-free days (VFDs) in the first 28 days after injury. Secondary outcomes included mortality, length of hospital stay, intensive care unit stay, and rates of complications (pneumonia, ventilator-associated pneumonia, sepsis, tracheostomy). RESULTS: A total of 207 patients were included in the analysis (operative group: 108 patients [52.2%]; mean [SD] age, 52.9 [13.5] years; 81 male [75%]; nonoperative group: 99 patients [47.8%]; mean [SD] age, 53.2 [14.3] years; 75 male [76%]). Mean (SD) VFDs were 22.7 (7.5) days for the operative group and 20.6 (9.7) days for the nonoperative group (mean difference, 2.1 days; 95% CI, -0.3 to 4.5 days; P = .09). Mortality was significantly higher in the nonoperative group (6 [6%]) than in the operative group (0%; P = .01). Rates of complications and length of stay were similar between groups. Subgroup analysis of patients who were mechanically ventilated at the time of randomization demonstrated a mean difference of 2.8 (95% CI, 0.1-5.5) VFDs in favor of operative treatment. CONCLUSIONS AND RELEVANCE: The findings of this randomized clinical trial suggest that operative treatment of patients with unstable chest wall injuries has modest benefit compared with nonoperative treatment. However, the potential advantage was primarily noted in the subgroup of patients who were ventilated at the time of randomization. No benefit to operative treatment was found in patients who were not ventilated. TRIAL REGISTRATION: ClinicalTrials.gov Identifier: NCT01367951.