Introduction Signet‐ring cell carcinoma is an extremely rare histological variant of upper urinary tract carcinoma, associated with poor prognosis. Case presentation We report a case of a 75‐year‐old female diagnosed with left primary upper urinary tract signet‐ring cell carcinoma, initially treated with surgery. Post‐surgical development of multifocal metastases was followed by a devastating clinical course. An autopsy confirmed the tumor's primary origin in the upper urinary tract. Conclusion We experienced a case of upper urinary tract signet‐ring cell carcinoma, with a rare opportunity to thoroughly confirm its primary site with an autopsy.