Background: The aim of this study is to investigate whether body mass index (BMI) is a prognostic factor in gastric cancer patients with peritoneal dissemination. Methods: This is a retrospective study consisting of 518 patients with a histological diagnosis of gastric cancer with peritoneal dissemination seen at the Sixth Affiliated Hospital of Sun Yat-Sen University and Sun Yat-sen University Cancer Center between January 2010 and April 2014. Patients were followed until December 2015. Chi-square test and Kaplan-Meier survival analysis were used to compare the clinicopathological variables and prognosis. Results: Univariate analyses showed that significant prognostic factors included palliative gastrectomy (p < 0.001), tumor size (p < 0.001), tumor location (p = 0.011), peritoneal seeding grade (p < 0.001), ascites (p = 0.001), serum CEA level (p = 0.002), serum CA19-9 level (p = 0.033), palliative chemotherapy (p < 0.001), and BMI group (p < 0.001). For patients with palliative chemotherapy, univariate analysis revealed that palliative gastrectomy (p < 0.001), tumor size (p = 0.002), tumor location (p = 0.024), peritoneal seeding grade (p = 0.008), serum CEA level (p = 0.041), and BMI group (p < 0.001). Multivariate analysis revealed that BMI was an independent prognostic factor in gastric cancer patients with peritoneal dissemination, especially in patients who received palliative chemotherapy. Conclusions: BMI is a prognostic factor for patients who have gastric cancer with peritoneal dissemination, especially in those who received palliative chemotherapy.