Background Computed tomography (CT) has become an important technique for assessing skeletal muscle mass. Low skeletal muscle mass (LSMM) is considered an unfavorable factor for postoperative complications in patients with gastric cancer (GC). Methods Patients who underwent laparoscopic gastrectomy for GC were included. Skeletal muscle mass at the third lumbar vertebra (L3) level was measured by preoperatively using CT. The patients were divided into an LSMM group and a non-LSMM group and the intergroup differences were analyzed. Furthermore, we divided the LSMM group into mild and severe LSMM subgroups. The study also analyzed the influence of obesity-related LSMM on postoperative complications. Results A total of 409 patients were enrolled; of them, 265 had LSMM and 41 had severe LSMM. LSMM was associated with age, body mass index, and Nutritional Risk Screening 2002 score. In the multivariate analysis, LSMM was not related to postoperative complications. Further analysis revealed that severe LSMM was a risk factor for postoperative complications. The study also found that the risk of postoperative complications was significantly increased in patients with obesity-related LSMM. Conclusions LSMM was not significantly correlated with postoperative complications. Severe LSMM and obesity-related LSMM are unfavorable factors for postoperative complications with GC after gastrectomy.