A 77-year-old man presented with decreased mental status and an enhancing partially cystic tumor along the left tentorium on magnetic resonance imaging after mastoidectomy and petrosectomy for an "auditory canal tumor." Smears of the aspirated cyst fluid revealed rare epithelial cell clusters, some with papillary features, foamy macrophages, and blood. The cells were orderly, with fairly bland nuclei and well-defined cell borders. The cell block contained similar epithelium, with cells containing eosinophilic and focally vacuolated cytoplasm, some with pigmented granules resembling hemosiderin. Numerous foam cells were also present. Review of the patient's previous and concurrent resection material showed an endolymphatic sac tumor, a rare neoplasm that arises in the endolymphatic sac in the temporal bone. The previously undescribed cytologic features of this rare neoplasm are discussed.