The case of a patient with gastric adenosquamous carcinoma with positive cancer cells on intraperitoneal washing cytology (CY1) who achieved a long recurrence-free survival is herein reported. A 74-year-old man was found to have adenosquamous carcinoma of the stomach. Partial gastrectomy was performed, and a pathological examination confirmed a diagnosis of adenosquamous carcinoma with invasion into the serosa and lymph node metastasis. S-1 monotherapy was administered because a cytologic examination revealed that the patient's peritoneal washings were positive for cancer cells. The patient remains alive with no recurrence two years and 10 months after undergoing surgery. Postoperative chemotherapy with S-1 monotherapy is effective for treating adenosquamous carcinoma of the stomach with CY1 and might contribute to long-term survival.