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Abstract Introduction In patients with gastric cancer, 6–27% of patients are diagnosed with T4b disease that invades adjacent organs, and curative resection can improve the prognosis of these patients. Case presentation A 70-year-old Japanese man presented with an abdominal tumor and was diagnosed with advanced gastric cancer (L-Circ type 3 T4b N2 M0 H0 stage IVA, based on the 15th edition of the Japanese Classification of Gastric Carcinoma) with extensive abdominal wall invasion. We performed open gastrojejunal bypass for gastric obstruction and initiated a chemotherapeutic regimen comprising S-1 (120 mg/day) and oxaliplatin (100 mg/m2). Upper gastrointestinal endoscopy performed after the administration of six courses of the S-1 and oxaliplatin regimen revealed a persistent primary lower gastric wall lesion; however, the diameter of the abdominal wall invasion and metastatic lymph nodes was significantly reduced, in addition to decreased serum carcinoembryonic antigen and carbohydrate antigen 19-9 levels. Subsequently, the patient underwent distal gastrectomy with D2 lymphadenectomy combined with transverse colon and abdominal wall resection. We performed radical en bloc resection and achieved a tumor-free resection margin. Simple abdominal wall closure was performed without mesh or musculocutaneous flap placement. Histopathological examination of the resected tumor specimen showed direct invasion of the mesocolon and rectus abdominis muscle. The patient was postoperatively diagnosed with L Gre-Ant type5 T4b (SI: rectus abdominis muscle) N2 PM0 DM0 Stage IIIA R0 Grade 2a gastric cancer based on histopathological findings and received S-1 as adjuvant chemotherapy, 2 months postoperatively. No recurrence was detected 6 months postoperatively. Conclusions We report a case of advanced gastric cancer with extensive abdominal wall invasion that was successfully treated with gastrectomy combined with resection of adjacent organs showing tumor invasion after effective systemic chemotherapy. A therapeutic approach comprising curative surgery combined with perioperative chemotherapy is useful in patients with T4b gastric cancer.