A 67-year-old woman with left cervical lymphadenopathy was referred to our hospital for further evaluation following a diagnosis of signet-ring cell carcinoma after a needle biopsy at a local medical doctor. The patient had an Eastern Cooperative Oncology Group Performance Status of 0. Her serum carbohydrate antigen (CA) 19-9 and carcinoembryonic antigen (CEA) levels were 16.8 U/ml and 2.5 ng/ml, respectively. Ultrasonography revealed three instances of lymph node enlargement in the left cervical area. Computed tomography (CT) showed left cervical lymphadenopathy and bilateral ovarian enlargement, but without specific signs of gastric cancer or lymph node metastasis surrounding the stomach. Esophagogastroduodenoscopy (EGD) revealed only slight atrophy of the stomach and there were no findings that indicated gastric cancer. Colonoscopy was also unremarkable. To make a definite diagnosis, we performed a lymphadenectomy of the left cervical region and pathological analysis revealed the presence of signet-ring cells and a poorly differentiated adenocarcinoma, which were suggestive of metastases originating from gastric cancer. However, fluorodeoxyglucose (FDG) positron emission computed tomography (PET–CT) showed no abnormal uptake, and although we could not detect a primary lesion at that point, we recommended chemotherapy pertinent to gastric cancer. The patient received monotherapy with oral S-1 (100 mg/body/day) for the first 4 weeks of a 6-week cycle, and after three courses of chemotherapy, CT showed a reduction in ovarian metastases without the appearance of new lesions; however, EGD continued to reveal no signs of gastric cancer. Although treatment with S-1 was effective, the patient complained of general fatigue, which was accompanied by an elevation in liver enzymes, and she was diagnosed as being allergic to S-1. Her chemotherapy regimen was switched to nab-paclitaxel (nab-PTX), which consisted of a 4-week course of intravenous nab-PTX (100 mg/body) on days 1, 8, and 15. While continuing this regimen for 18 months, the bilateral ovarian metastases remained stable. As there was no evidence of other lesions, including in the stomach, we performed a bilateral oophorectomy. There were no remarkable changes in gastric serosa and surrounding tissues of stomach and microscopic examination of the specimen confirmed a diagnosis of metastatic adenocarcinoma that consisted of a signet-ring cell carcinoma and a poorly differentiated adenocarcinoma. These findings again suggested the presence of a primary gastric lesion. The patient was carefully followed up with continued chemotherapy (nab-PTX). At 3 months after the oophorectomy, we detected a limited rough-surfaced mucosa with slight redness near the pyloric ring that stained positive for indigo carmine during endoscopic examination without any abnormality in other areas of antrum and gastric body. Biopsy specimens revealed a poorly differentiated adenocarcinoma with signet-ring cells, and this was considered as evidence of gastric cancer. Nonetheless, CT showed no specific changes in the stomach or in the nearby lymph nodes and PET–CT also showed no abnormal uptake in the whole body. We discussed the possibility of a R0 resection and decided to perform conversion surgery. We discussed the method of surgery, and decided to proceed with distal gastrectomy considering the postoperative nutrition and absence of obvious signs that indicate the extent of the cancer in the upper area. Thus, a definitive diagnosis could be established only about 2 years after the initial diagnosis of CUP and the patient underwent a distal gastrectomy with D2 lymphadenectomy. Detecting the cancerous area based on macroscopic findings was challenging and we did not find lymph node enlargement or peritoneal metastases intraoperatively. Pathological evaluation was graded as type 5 with T3 invasion that consisted of a poorly differentiated adenocarcinoma with signet-ring cells. The histological response of the primary and lymphatic tumor was grade 1a. Although the cancerous tissue was spread over a wide area with multiple lymph node metastases (27/32) (N3b), unexpectedly, both proximal and distal margins and the peritoneal washing were negative for cytology, according to the Japanese Classification of Gastric Carcinoma []. Her postoperative course was uneventful, and the patient was discharged on postoperative day 12. Currently, the patient is alive with no sign of disease recurrence at 3 months post-surgery.