Nivolumab, a monoclonal antibody targeting programmed cell death-1, significantly prolongs survival for patients with advanced non-small-cell lung cancer (NSCLC). However, little is known about the value of predictive biomarkers. Hence, we investigated the impact of skeletal muscle (SM) mass loss on clinical outcomes in NSCLC patients undergoing nivolumab treatment. Thirty patients with histologically confirmed NSCLC treated with nivolumab were included in this study. Computed tomography was used to determine SM loss based on the SM index (SMI). The SMI is the cross-sectional area of the bilateral psoas muscles at the third lumbar vertebra, divided by height squared. The cut-off values were defined as 6.36 cm/m for men and 3.92 cm/m for women. Among the 30 patients, 13 (43%) had SM loss. There was no significant association between SM loss and immune-related adverse events. The SM loss group had undergone significantly more prior chemotherapy cycles (P = .04). SM loss was significantly associated with fewer nivolumab cycles (P = .01). No patients in the SM loss group achieved a partial response. Patients with SM loss had a significantly shorter progression-free survival period (P = .008) and median overall survival than those with normal SM mass (10 vs 25 months, respectively, P = .03). SM loss was an independent prognostic factor of poor survival. In conclusion, SM loss may be a predictive factor of poor outcomes in NSCLS patients undergoing nivolumab therapy.