A 12-year-old Belgian Warmblood mare was referred to the equine clinic of the Faculty of VeterinaryMedicine (Ghent University) for a mass on the third eyelid of the right eye. The horse had been having arecurrent red-tinged ocular discharge for several months. After complete ophthalmologic examination, a largepart of the third eyelid was surgically excised and submitted for histopathologic examination. Thehistopathologic examination revealed a proliferation of blood-filled vascular spaces lined by a single layer offlattened cells, which were identified as endothelial cells using immunohistochemistry. The tumor wasidentified as a cavernous low-grade hemangiosarcoma. No complications were reported six months aftersurgery.