Pseudomyxoma peritonei, a rare condition consisting of intraperitoneal mucinous tumors and ascites, most commonly arises from mucinous tumors of the appendix. Very rarely, mucinous deposits arise in the retroperitoneum without intraperitoneal involvement. This has been termed pseudomyxoma extraperitonei. It is a rare and poorly understood condition that is heterogeneous in its clinical behavior, and only a few cases presenting as localized disease in the retroperitoneum have been reported. In this paper, we report the first case of pseudomyxoma extraperitonei presenting as a simple renal hilar mass and mimicking a tumor of renal origin in a horseshoe-kidney patient. The patient underwent isthmusectomy and nephrectomy. Immunohistochemical staining suggested appendiceal origin. She remained alive without adjuvant therapy postoperatively, and no evidence of recurrence was present for 25 months.