Primary mucinous cystic neoplasms are extraordinarily rare tumors of the kidney. Herein, we present a case of a 52-year-old man who presented with painless hematuria and mucusuria. The nephrectomy showed a markedly enlarged kidney replaced by a large cystic mass filled with mucin and with almost complete destruction of the renal parenchyma. Histologically, the mass was lined primarily by simple mucinous epithelium and showed foci of adenomatous (borderline) change with focal areas exhibiting intraepithelial carcinoma. No invasion was documented after extensive sampling of the tumor. A pathologic diagnosis of intraepithelial carcinoma arising in a mucinous neoplasm of borderline malignancy was rendered. One year later, the patient presented with pseudomyxoma peritonei. This case illustrates that, in a patient with pseudomyxoma peritonei, the absence of an appendiceal, gastrointestinal, or ovarian primary tumor raises the possibility of a primary neoplasm at an unusual site such as the kidney.