Background: Weight loss during chemotherapy is a significant prognostic factor for poor survival in patients advanced gastric cancer (AGC). However, in most studies, weight loss was measured at the end of chemotherapy, limiting its clinical use. In this study, we evaluated whether weight loss during the first month of chemotherapy could predict survival outcomes in patients with AGC. Methods: We analyzed 719 patients with metastatic or recurrent AGC who were receiving palliative chemotherapy. We calculated initial body mass index (BMIi), percent weight loss after one month of chemotherapy (ΔW1m), percent weight loss after last administration of chemotherapy (ΔWend), and average weight loss per month during chemotherapy (ΔW/m). We correlated these data with overall survival (OS) by receiver operating characteristic (ROC) curves and Kaplan-Meier curves, and performed a subgroup analysis using Cox regression. Results: The probabilities of longer OS had stronger correlations with ΔW/m and ΔW1m than with ΔWend or BMIi. The optimal cutoff values of ΔW/m and ΔW1m for predicting shorter survival were 1% and 3%, respectively. A significant positive correlation between ΔW1m and ΔW/m (r² = 0.591, p < 0.001) was observed. Patients with ΔW1m more than 3% significantly younger, had worse performance status, more diffuse-type Lauren classification, more HER2-negative pathology, a higher number of involved organs, and more peritoneal seeding at initial presentation. OS of patients with ΔW1m more than 3% were significantly shorter than patients with less weight loss (ΔW1m ≥3%: 9.7, <3%: 16.3 months, p < 0.001). Patients who recovered average weight loss per month after experiencing weight loss at the first month showed prolonged OS compared with patients who did not recovered (ΔW/m < 1%: 21.3, ≥1%: 7.8 months, p < 0.001). Subgroup analysis revealed ΔW1m accompanied poor survival irrespective of other clinical characteristics. Multivariate analysis showed weight loss at the first month of chemotherapy adversely affected OS (p= 0.038). Conclusions: Weight loss at the very first month of palliative chemotherapy could predict unfavorable survival outcomes in AGC.