To assess the suitability of the 7th AJCC/UICC TNM staging system in predicting the prognosis of synchronous multiple gastric carcinomas (SMGCs). A total of 129 SMGC patients who underwent gastrectomy with D2 lymphadenectomy from January 1999 to January 2009 were enrolled in this study. The location, diameter, and depth of invasion of the main tumor were all related to prognosis (P < 0.05). Multivariate analysis revealed depth of invasion as an independent predictive factor for survival (P < 0.05). Interestingly, logistic regression analysis showed that the 7th AJCC/UICC N staging system was unable to significantly predict survival in SMGCS patients (P > 0.05). Cut-point survival analysis identified the most appropriate cut-offs for metastatic lymph nodes (MLNs) as 0, 1, 6, 10, and 19: patients with 0, 1-6, 7-10, and 11-19, and ≥ 20 MLNs had median survival times of 70, 56, 35, 52, and 32 months, respectively. Multivariate analysis suggested this new categorization of MLNs to be a significant predictor of survival (P < 0.05). Preoperative assessment of depth of invasion can help in the prognosis of SMGCs patients. The 7th UICC TNM staging system may be not suitable for SMGC patients and needs improvement for rational grading of SMGCs.