Low skeletal muscle density (SMD) and low skeletal muscle index (SMI) are associated with poor overall survival (OS) in patients with various types of cancer. We retrospectively studied SMD and SMI using computed tomographic (CT) scans in patients with gastric cancer receiving chemotherapy to evaluate its prognostic significance. SMD and SMI were obtained from CT-based analysis using Slice-O-Matic® medical imaging software in patients who received S-1 plus cisplatin chemotherapy for metastatic gastric cancer. The CT images taken within 1 month before starting chemotherapy were used. The cut-off values for determining low SMD [<33 Hounsfield units (HU) in obese and <41 HU in non-obese patients] and low SMI (<41 cm2/m2 in females, <43 cm2/m2 in non-obese males and <53 cm2/m2 in obese males) were referenced from a large population based study. The CT images of 53 patients were reviewed. The median SMD was 36.8 HU (range, 19.5-59.3 HU), and the median SMI was 39.8 cm2/m2 (range, 23.7-60.0 cm2/m2). Patients with low SMD had significantly shorter OS compared with patients having normal SMD (8.9 vs. 12.8 months, P=0.03). However, OS did not differ significantly between patients with low and normal SMI (11.1 and 14.3 months, P=0.18). Multivariate analyses confirmed that low SMD was an independent predictor of poor outcomes (P<0.01). SMD is an important prognosticator of survival in patients with metastatic gastric cancer receiving chemotherapy.