An 11-year-old female tokay gecko (Gekko gecko) was presented for bilateral asymmetric swellings of the cervical region. Baseline hematologic and plasma biochemical analyses demonstrated anemia, hypercalcemia, hyperphosphatemia, and hypoalbuminemia. Whole-body radiographs revealed a mass effect and irregular mineralization in the region of the left endolymphatic sac. Fine needle aspiration of this region revealed many elongated calcium-based crystals with pointed ends, histiocytic inflammation, and clusters of epithelial cells, consistent with hyperplasia or neoplasia of the epithelium of the endolymphatic sac. The crystals were shown to be calcium carbonate by infrared crystallography. Given the advanced state of disease and inability to completely excise the mass due to the intimate association between the meninges and endolymphatic sac, the gecko was euthanized. Gross necropsy and histopathologic evaluation revealed carcinoma in situ of the left endolymphatic sac. To our knowledge, this is the first report of neoplasia of the endolymphatic sac in a reptile.