Abstract Background The mortality rate of patients with unresectable gastric cancer (UGC) has decreased with the development of chemotherapies and surgical techniques. However, the survival rate remains low. We retrospectively examined the prognostic significance of the pretreatment skeletal muscle mass index (SMI) and nutritional and inflammatory factors in patients with UGC. Methods This study included 83 patients diagnosed with UGC at Tottori University Hospital who received palliative chemotherapy based on 5-fluorouracil. Pretreatment computed tomography (CT) measured overall skeletal muscle mass (SMM) and cross-sectional SMM at the third lumbar vertebra (L3). We focused on the neutrophil-to-lymphocyte ratio (NLR), C-reactive protein-to-albumin ratio (CAR), prognostic nutritional index (PNI), and platelet-to-lymphocyte ratio (PLR) as nutritional and inflammatory factors. Results Receiver operating characteristic curve analysis was performed for median survival time (MST) after palliative chemotherapy. SMIs for males and females (43.9 cm2/m2 and 34.7 cm2/m2, respectively) were the cutoff values, and patients were divided into high (SMIHigh; n = 41) and low SMI groups (SMILow; n = 42). Body mass index (BMI) was significantly higher in patients in the SMIHigh group than in the SMILow group (p