Objective This study aimed to determine the risk of severe postoperative complications (SPCs) in patients with gastric cancer and to construct a nomogram based on independently related factors to identify high-risk patients. Methods We conducted a prospective study of 636 consecutive patients with gastric cancer who underwent radical gastrectomy. Degrees of sarcopenia and obesity were calculated before surgery. Factors contributing to SPCs were determined using univariate and multivariate analysis. A nomogram consisting of the independent risk factors was constructed to quantify the individual risk of SPCs. Results Logistic analysis revealed that sarcopenic obesity, age, open surgery, and combined resection were independent prognostic factors for SPCs. Sarcopenic obese patients have the highest risk in all patients (sarcopenic obesity vs normal, OR = 6.575 p  = 0.001; sarcopenic obesity vs obesity, OR = 5.833 p  = 0.001; sarcopenic obesity vs sarcopenia, OR = 2.571 p  = 0.032), while obese patients share the similar rate of SPCs with normal people (obesity vs normal, OR = 1.056 p  = 0.723). The nomogram we constructed was able to quantify the risk of SPCs reliably (c-index, 0.737). Conclusions Sarcopenic obesity, together with age, open surgery, and combined resection are independent predictors of SPCs. Obesity will significantly increase the risk of SPCs in sarcopenic patient with gastric cancer, but it will not bring higher risk to normal patients. Our nomogram is a simple and practical instrument to identify patients at high risk of surgical complications.