Purpose Preoperative nutritional and immunological status is closely associated with postoperative outcomes in gastric cancer. Although osteopenia and sarcopenia have been reported as prognostic factors, the significance of their coexistence as osteosarcopenia and its value relative to established nutritional indices remain unclear. This study aimed to clarify the prognostic impact of osteosarcopenia in patients undergoing curative gastrectomy.Methods This study included 273 patients who underwent curative gastrectomy for gastric cancer. Osteopenia and sarcopenia were assessed using preoperative computed tomography–derived bone mineral density and skeletal muscle index, respectively, and osteosarcopenia was defined as their coexistence. Survival analysis was performed using the Kaplan-Meier method and Cox proportional hazards models, and prognostic predictive ability was evaluated by time-dependent ROC analysis.Results Osteopenia, sarcopenia, and osteosarcopenia were observed in 40.7%, 49.5%, and 22.3% of patients, respectively. Osteopenia and osteosarcopenia were independent predictors of poor OS and RFS, whereas sarcopenia alone was not. Osteosarcopenia was associated with older age, systemic inflammation, and poor nutritional status. A composite Cox model–based risk score integrating body composition and nutritional parameters demonstrated superior prognostic accuracy.Conclusion Preoperative osteosarcopenia is a strong prognostic indicator in gastric cancer, and multidimensional assessment improves survival prediction.