Background: Sarcopenia is a progressive condition characterized by loss of skeletal muscle mass, muscle strength, and physical function, and is identified as an important prognostic factor in malignant tumors. The patients of gastrointestinal (GI) malignancies are often poor nutrition, weight loss, and loss of skeletal muscle mass due to pathophysiology such as the dysfunction of nutritional absorption and GI obstruction. The prevalence of sarcopenia is high in GI malignancies, especially in advanced gastric cancer (GC), which is reported to be 30%. Recently, in assessing sarcopenia, muscle quality as well as skeletal muscle mass has been considered important and some studies suggest that intramuscular adipose tissue is useful in assessing muscle quality. In this study, we focused on the fat ratio within psoas muscle (FRPM) and sought to clarify the impact of FRPM on overall survival (OS) in GC patients undergoing systemic chemotherapy. Methods: We retrospectively analyzed the patients with histologically confirmed stage IV GC receiving at least one systemic chemotherapy between February 2017 and March 2021 in our hospital. We measured PMI (Psoas Muscle Index: bilateral muscle mass (cm²)/height squared(m²)) and FRPM both at the lumber three level on the CT images before chemotherapy. PMI and FRPM values were classified into two groups, High and Low, according to whether they were higher or lower than the median, and differences in OS were analyzed for each group. Results: The analysis included 79patients (median age: 69 years, 59 males). The median FRPM was 1.67%. The FRPM-high group and the FRPM-low group were 40/39patients, respectively. The median PMI in male and female patients was 4.35 cm²/m² and 2.88 cm²/m². Thirty male patients and ten female patients were defined as the PMI-high group, and the remaining 39 patients were in the PMI-low group. The 1, 2 and 3year cumulative OS rate in patients with FRPM-high and FRPM-low was 67.3%, 14.3% and 7.6% in the FRPM-high group and 74.8%, 40.5% and 32.4% in the FRPM-low group (P = 0.0341). The 1, 2 and 3year cumulative OS rate in patients with PMI-high and PMI-low was 86.7%, 40.4% and 30.0% in the PMI-high group and 55.8%, 12.8% and 6.4% in the PMI-low group (P< 0.0001). In the multivariate analysis of factors associated with OS, PMI (P = 0.0047) and FRPM (P = 0.0019) were independent predictors for the OS. Conclusions: High-FRPM may be associated with declined physical activity, suggesting that skeletal muscle function may be an important prognostic factor in patients with GC receiving systemic chemotherapy.