B-scan ultrasonography of the right eye of a 6-year-old sexually intact female mixed-breed dog revealed complete retinal detachment and a semiconical solid mass of tissue that projected from the area of the optic disc into the vitreal space subretinally. Microscopic examination of an ultrasound guided fine-needle aspirate of the mass revealed a moderate number of mildly pleomorphic melanocytes. Histologic examination of the excised globe revealed a large, heavily pigmented intraocular neoplastic mass in the choroid around the optic nerve, bulging into the subretinal space. The histologic diagnosis was benign choroidal melanoma, with invasion into the intraocular portion of the optic nerve. In dogs, tumors of melanocytic origin are the most common primary ocular neoplasm, but primary choroidal melanocytic neoplasms are the rarest of this type.