The aim of this study was to investigate the effect of the body fat mass ratio on survival and prognosis in advanced non-small-cell lung cancer patients. The study includes 200 patients who were diagnosed with advanced non-small-cell lung cancer between 2014 and 2018 and whose body fat mass percentage and body mass index (BMI) were determined using the Tanita Body Composition Analyzer during admission. All patients had advanced incurable non-small-cell lung cancer (30% had locally advanced disease, 70% were stage IV). In the univariate and multivariate analyses, age, gender, histopathological type, smoking history, comorbidities, weight loss in the last six months and body mass index had no statistically significant effect on survival (p > 0.05). However, the performance status (p = 0.008), metastatic status (p = 0.003) and body fat mass ratio (p = 0.01) were found to have a significant effect on overall survival (OS): the median OS was 16.4 mo, in patients with the BFM ratio ≤ 22% and 29.2 mo, in those with > 22% (p = 0.01). In this study, it was found that the body fat mass ratio was an important prognostic factor in patients with advanced non-small-cell lung cancer.