Malnutrition and cachexia occur commonly in patients with advanced gastric cancer (AGC). This study elucidated the effect of nutritional support (NS) on survival outcomes among patients with AGC undergoing chemotherapy. We retrospectively evaluated new AGC cases at our institute between January 2015 and January 2021. Inclusion criteria were unresectable or recurrent chemotherapy-treated gastric adenocarcinoma, ECOG performance status (PS) 0-2, and adequate organ function. Time to treatment failure (TTF) and overall survival (OS) were evaluated, and univariate and multivariate analyses identified prognostic factors. A total of 103 eligible patients were separated into groups: 69 patients (67%) into NS and 34 (33%) into routine care (RC). The median follow-up time was 11.0 mo, (0.5-92). NS was offered to patients with poorer PS (p = 0.03), Glasgow prognostic score (GPS) positivity (p = 0.001), and high neutrophil-to-lymphocyte ratios (cut-off ≤ 3, p = 0.02). Median OS and TTF in the RC and NS groups were 11.6 and 10.4 mo, (p = 0.99) and 4.2 and 5.5 mo, (p = 0.07), respectively. Multivariate analyses identified NS (hazard ratio [HR] = 0.53, p = 0.01) and GPS positivity for TTF, and low body mass index (HR = 2.03, p = 0.007) and GPS positivity (HR = 2.25, p = 0.001) for OS as significant prognostic factors. Thus, NS with chemotherapy is a potentially effective intervention for AGC.