Primary splenic mucinous cystadenocarcinoma (MCCa) is extremely rare, and only six cases appear to have been reported previously. We present herein a case of primary splenic MCCa resulting in pseudomyxoma peritonei (PMP). A 66-year-old Japanese woman presented to a hospital with a chief complaint of upper abdominal pain and a 7-year history of splenic cyst. Cyst rupture was noted on computed tomography, and splenectomy was performed. The abdominal cavity was filled with a large amount of gelatinous ascites, with the appearance of PMP. On the cut surface, multiple cysts containing mucinous material were found within and outside the spleen. Microscopically, splenic parenchyma was occupied by large mucinous pools focally lined with mucinous epithelial cells and mesothelial cell-like cells, which were considered benign. Outside the spleen, a low-grade MCCa component was found. No ectopic pancreatic or intestinal tissue was identified. Although most PMP cases are known to be caused by low-grade mucinous appendiceal tumor, the present case represents the first report of a splenic MCCa resulting in PMP.