An adult discus (Symphisodon aequifasciatus) with a history of buphthalmia and anorexia was referred, from an aquarium, for clinical examination. Surgery was performed to enucleate the eye and obtain a definitive diagnosis. Macroscopic examination revealed a yellowish mass that affected the eye. Histologic examination of the mass revealed a poorly demarcated, high cellular infiltrative mesenchymal neoplasm composed of ovoidal-spindloid cells with elongated nuclei and with scant cytoplasm arranged in a solid pattern. Throughout the tumor, there were scattered foci of cartilaginous differentiation that stained deeply with alcian blue and stained metachromatically with toluidine blue. The neoplastic cells were immunolabeled to varying degrees by antibodies against vimentin and SOX-9. Based on morphologic, histochemical, and immunohistochemical characteristics, the tumor was diagnosed as a chondrosarcoma.