The treatment of stage III non-small cell lung cancer (NSCLC) is complex. Here, we aimed to examine the prognostic utility of sarcopenia and metabolic muscle volumes and evaluate their relationship with oncological treatments in patients with locally advanced NSCLC. Patients with unresectable stage III NSCLC were evaluated retrospectively. Muscle fields were measured, and metabolic parameters of the psoas were obtained. The skeletal muscle index (SMI), sarcopenia, sarcopenic obesity, and body mass index (BMI)-associated sarcopenia were evaluated. Fifty-three (94.6%) patients were men, and three (5.4%) were women. Sarcopenia was identified in 36 (64.3%) patients. Pretreatment sarcopenia and BMI-associated sarcopenia negatively affected overall survival (p = 0.040 and 0.023, respectively). A high psoas SUVmean (Standardized Uptake Value mean) and low mean psoas HU (Hounsfield unit) were poor prognostic factors (p = 0.009 and 0.014, respectively). SMI and muscle mass decreased after oncological treatment. Advanced age, inability to complete treatment, administration of chemoradiotherapy after chemotherapy, presence of sarcopenia, and a low mean psoas HU decreased survival. In conclusion, sarcopenia and BMI-associated sarcopenia are poor prognostic factors in patients with lung cancer. Oncological treatments can adversely affect muscle mass. The metabolic parameters of the psoas muscle can predict patient prognosis.