Background: The aim of this study was to examine the effect of a perioperative oral nutritional supplement in malnourished patients who undergo gastrectomy. Methods: Patients who were determined as being moderately or severely malnourished according to a patient-generated subjective global assessment or who had a body mass index <18.5, were enrolled. The oral nutritional supplement group received 500 mL/d of standard oral nutritional supplement for 2 weeks before gastrectomy and for 4 weeks postoperatively. The primary endpoint was postoperative complications (Clavien-Dindo classification ≥II). The secondary endpoints included body weight changes, biochemical parameters, and quality of life survey results. Results: A total of 127 patients (65 in the oral nutritional supplement group and 62 in the control group) were enrolled. The complication rates were not significantly different (29.2% versus 37.1%, P =.346). However, the incidences of overall complications, complications persisting until postoperative week 3 or 5, and severe complications (grade ≥IIIa) were significantly lower in the oral nutritional supplement group for patients with patient-generated subjective global assessment grade C. Total lymphocyte counts were significantly higher in the oral nutritional supplement group at postoperative weeks 3 and 5. For most patients, oral nutritional supplement was well tolerated preoperatively. However, only 26.2% and 50.8% of the patients in the oral nutritional supplement group could consume >250 mL/d of oral nutritional supplement postoperatively during the 2nd and 4th weeks, respectively. Conclusions: The routine application of perioperative oral nutritional supplement is not recommended for malnourished patients receiving gastrectomy. However, perioperative standard oral nutritional supplement administration may reduce the incidence, severity, and duration of complications after gastrectomy in severely malnourished patients (patient-generated subjective global assessment grade C).