Introduction The increased visceral fat in patients with obesity can increase the technical difficulty of surgery. This study was performed to evaluate a preoperative 20‐day very low‐calorie diet for obesity before laparoscopic gastrectomy for gastric cancer. Methods This prospective single‐center study involved patients with obesity who were planning to undergo laparoscopic gastrectomy for gastric cancer. Obesity was defined according to the Japanese criteria: BMI ≥25 kg/m2 or waist circumference ≥85 cm in men and ≥90 cm in women. The patients underwent a preoperative 20‐day very low‐calorie diet and received nutritional counseling. Weight loss, body composition, visceral fat mass, and operative outcomes were evaluated. Results Thirty‐three patients were enrolled from September 2013 to August 2015. Their median age was 71 years, and 78.8% were men. Their median bodyweight and BMI were 72.3 kg (range, 53.8–82.5 kg) and 26.0 kg/m2 (range, 23.5–31.0 kg/m2), respectively. The patients achieved a mean weight loss of 4.5% (95% confidence interval [CI]: 3.8–5.1), corresponding to 3.2 kg (95%CI: 2.7–3.7 kg). Body fat mass was significantly decreased by a mean of 2.5 kg (95%CI: 1.9–3.1), whereas skeletal muscle mass was unaffected (mean: −0.20 kg [95%CI: −0.55–0.15]). The visceral fat mass reduction rate was high as 16.8% (range, 11.6%–22.0%). All patients underwent laparoscopic gastrectomy as planned. Severe postoperative morbidity (Clavien–Dindo grade ≥III) was seen in only one patient (3.0%). Conclusion The preoperative 20‐day very low‐calorie diet weight loss program is promising for the treatment of obesity before laparoscopic gastrectomy for gastric cancer.