Background: Malnutrition after gastrectomy is associated with a poor prognosis; however, no accurate model for predicting post-gastrectomy malnutrition exists. Hence, we conducted a retrospective study to develop a prediction model identifying gastric cancer patients at high risk of malnutrition after gastrectomy. Method: Gastric cancer patients who underwent curative gastrectomy with more than one weight measurement during a 3-year follow-up period were included. Malnutrition was defined as body mass index (BMI) < 18.5 kg/m2 according to the European Society of Clinical Nutrition and Metabolism diagnostic criteria. BMI-loss pattern was analyzed using a group-based trajectory model. A prediction model for malnutrition 6 months after gastrectomy was developed based on significant risk factors, and then validated. Results: Overall, 1421 patients were examined. The BMI-loss trajectory model showed significant BMI loss at 6 months after gastrectomy. Severe BMI loss (mean 21.5%; n = 109) was significantly associated with the elderly, female sex, higher preoperative BMI, advanced cancer stage, open surgery, total gastrectomy, Roux-en-Y reconstruction, chemotherapy, and postoperative complications (all p < 0.05). Malnutrition 6 months after gastrectomy was observed in 152 (11.9%) of 1281 patients. Preoperative BMI, sex, and type of operation were included in the final prediction model as predictive factors (p < 0.05). The C-index of the developmental set and bootstrap validation of the prediction model was 0.91 (95% confidence interval 0.89–0.94) and 0.91, respectively. Conclusion: The prediction model for the risk of malnutrition 6 months after gastrectomy was accurately developed, with three independent risk factors: low preoperative BMI, female sex, and total or proximal gastrectomy.