Primary signet-ring cell carcinoma of the prostate is infrequent and even more so as secondary spread of this pathologic sub-type to the prostate. We describe the sixth reported case with a diagnosis of a secondary signet-ring cell tumour of the prostate secondary to a gastric cancer. Five years post-gastrectomy to resect signet-ring cell carcinoma, we detected a secondary intra-prostatic spread with urinary tract obstruction. The physical appearance of the tumour cells was similar to that of the previously-resected signet-cell carcinoma of the stomach. There were no metastases in other sites and the patient was treated with radiotherapy. When confronted with intra-prostatic signet-ring cell adenocarcinoma it is necessary to distinguish between primary and secondary aetiology since this would reflect in the choice of treatment and prognosis.