Background The intra-abdominal cavity, surrounded by adipocytes, is the main metastatic site of epithelial ovarian, fallopian tube, and peritoneal cancer. Epidemiological and molecular studies have demonstrated a link between adipose tissue and ovarian cancer. However, the clinical significance of fatty tissue has not been elucidated. Thus, we investigated the clinical significance of body composition in patients with epithelial ovarian, fallopian tube, and peritoneal cancer. Methods Fat and skeletal muscle areas were measured using software based on pretreatment computed tomography scans at the third lumbar vertebra. Fat-to-muscle ratios were calculated using the total (visceral and subcutaneous) fat area or visceral fat area. High fat-to-muscle ratios were defined by values greater than the mean. Sarcopenia was defined as a skeletal muscle index < 38.7 cm2/m2. The clinicopathological parameters and survival of 153 patients were analyzed. Results High visceral fat-to-muscle ratios and sarcopenia at the time of diagnosis were observed in 43.8% and 33.3% of the patients, respectively. Multivariate analysis showed that high visceral fat-to-muscle ratio (p = 0.014), advanced Federation of Gynecology and Obstetrics stage (p = 0.008), and chemoresistance (p = 0.027) were independent factors for worse overall survival. Patients with high visceral fat-to-muscle ratios were older, had higher body mass indexes, and were more likely to have diabetes/hypertension, serous cancer subtypes, and implementation of neoadjuvant chemotherapy than those with low visceral fat-to-muscle ratios. The platelet count was significantly higher in the high visceral fat-to-muscle ratio group than in the low visceral fat-to-muscle ratio group (p = 0.011). Conclusions Pretreatment visceral fat area could be an independent predictive factor of overall survival in patients with epithelial ovarian, fallopian tube, and peritoneal cancer and may be significantly associated with thrombocytosis.