Cancer of unknown primary accounts for approximately 3 – 5% of all malignancies and is typically associated with a dismal prognosis. We describe a 65-year-old man who presented with skeletal metastasis and circulating tumor cells exhibiting signet ring (SR) morphology. The patient was diagnosed with SR cell carcinoma (SRCC) through a bone marrow biopsy. This case report aimed to emphasize the importance of clinicians’ awareness of SRCC of the urinary bladder. The primary site of tumor origin was not identified as antemortem. The patient died 2 months after being admitted for pulmonary embolism. At autopsy, the urinary bladder was determined to be the primary site of the tumor. Primary SRCC of the urinary bladder is extremely rare. There are currently no established consensus guidelines for its management. Surgery continues to be the primary treatment option when the condition is localized.

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