Recently, zolbetuximab combined with chemotherapy has been approved as a first-line treatment for advanced gastric cancer (GC). However, to date, no endoscopic images demonstrating endoscopic complete response (eCR) to zolbetuximab plus chemotherapy have been reported. Herein, we report the case of an 80-year-old man who presented with epigastric pain and loss of appetite and was diagnosed with undifferentiated GC and peritoneal metastasis. The patient received modified FOLFOX6 plus zolbetuximab because genetic testing revealed HER2-negative and claudin 18.2-positive. He experienced grade 2 nausea, which improved with the administration of antiemetic agents. Computed tomography performed at month 5 showed resolution of peritoneal dissemination. Endoscopic examination during the same period showed the disappearance of the primary gastric tumor with scarring. Biopsy of the scar area revealed only inflammatory cells; therefore, the primary lesion was defined as an eCR. To the best of our knowledge, this is the first report of eCR to zolbetuximab and chemotherapy in a patient with GC and claudin 18.2 positive. This case report suggested that chemotherapy combined with zolbetuximab may offer greater therapeutic benefit than chemotherapy combined with immune checkpoint inhibitors in patients with undifferentiated GC. Further analysis is expected to enable the identification of patients with claudin 18.2-positive GC in whom zolbetuximab treatment is effective.