This prospective study was undertaken to investigate whether preoperative oral nutritional supplementation (ONS) would increase the prognostic nutritional index (PNI) in gastric cancer patients undergoing gastrectomy. Before surgery for resectable gastric cancer, Racol® NF (Otsuka Pharmaceutical Factory, Japan) was administered orally at a recommended dose of 600 kcal/600 ml per day to patients with a PNI of <48. The primary endpoint was the change in the PNI, which was calculated as 10 × serum albumin (g/dl) + 0.005 × total lymphocyte count (per mm3). Forty-six patients received Racol® NF. The mean PNI at baseline and before surgery was 44 ± 3.9 and 43 ± 4.4, respectively (p = 0.049). The mean serum albumin level was significantly decreased after the administration of Racol® NF (p = 0.001), while the mean total lymphocyte count (p = 0.001) and body weight (p = 0.004) were significantly increased. The mean daily intake and duration of Racol® NF administration were 340 ml and 22.6 day, respectively. Adverse events during the administration of Racol® NF were observed in 12 (26.1%) patients. The present study indicated that preoperative ONS did not increase the PNI in gastric cancer patients with low PNI values.