Limited information is available regarding the impact of body weight loss (BWL) in patients with advanced gastric cancer (AGC) who receive second-line chemotherapy. We retrospectively reviewed data for consecutive AGC patients who received second-line treatment with taxane-based chemotherapy at our institution between January 2014 and September 2018. We calculated variables, including percent BWL per month during chemotherapy (%BWL/m), and analyzed the correlations between BWL and other clinicopathological parameters with survival. Forty-four AGC patients were registered (median age, 67.5 years; females, n = 16 [36.3%]; severe ascites, n = 12 [27.3%]). The median overall survival was significantly shorter among patients with a %BWL/m of 1% or more, compared with patients with less weight loss (6.3 mo, vs. 12.3 mo, P = 0.038). The %BWL/m (≥1% vs. <1%) was significantly correlated with survival in a univariate analysis (HR = 2.11, P = 0.04), and the survival period was shorter for patients with severe ascites (HR = 1.92; 95% CI, 0.90-3.90) and if their %BWL/m was 1% or more (HR = 2.01; 95% CI, 0.98-4.10) in a multivariate analysis. In conclusion, BWL during second-line chemotherapy was associated with a poor prognosis among patients with AGC.