Sarcopenia represented by low psoas muscle area is associated with increased hospital length of stay (LOS), postoperative complications, and mortality. We studied whether thoracic skeletal muscle area (TSMA) derived from computed tomography (CT) predicts morbidity after pneumonectomy for lung cancer. Consecutive patients who underwent pneumonectomy for lung cancer from 2005 to 2017 were retrospectively analyzed. TSMA was defined as the sum of muscle area at the level of the eighth and the 12th thoracic vertebral bodies on preoperative CT. Patients were stratified into sex-specific TSMA quartiles for univariate time-to-event analyses. The effect of continuous TSMA measurements on operative complications, hospital and intensive care unit (ICU) LOS, discharge disposition, and hospital readmission within 90 days was estimated using multivariable models adjusted for age, sex, body mass index, forced expiratory volume in 1 second, Zubrod score, and pneumonectomy type. Standard (n = 102, 78.5%) or high-risk (n = 28, 21.5%; extrapleural: n = 3, 2.3%; carinal: n = 9, 6.9%; completion: n = 16, 12.3%) pneumonectomy was performed in 130 patients (60.8 ± 10.6 years; 43.1% women). Major complications occurred in 33.1% (n = 43 of 130) and readmission in 17.7% (n = 23 of 130) of patients. In multivariable models, patients with high TSMA experienced fewer overall (odds ratio OR, 0.87; P = .04) and cardiopulmonary (OR, 0.86; P = .04) complications, and fewer readmissions (OR, 0.78; P = .01). Associations with ICU LOS (hazard ratio, 1.08; P = .051) and hospital LOS (hazard ratio, 1.05; P = .18) did not reach significance. TSMA predicts adverse outcome after pneumonectomy for lung cancer. This marker, readily derived from standard chest CT, identifies patients at increased risk for postoperative complications and may help select patients appropriate for focused rehabilitation before pneumonectomy.