A 14-year-old male adolescent with ocular melanocytosis and secondary glaucoma in the left eye had a 2-year history of a progressively enlarging fundus lesion. Ocular examination revealed diffuse hyperpigmentation of the episclera and a smooth velvety thickening and hyperpigmentation of the left iris. Ophthalmoscopy disclosed diffuse choroidal pigmentation and a pigmented mass that occupied the macular area and surrounded the optic nerve. Ultrasonography showed an acoustically hollow lesion with scleral bowing and choroidal excavation. Based on clinical and ultrasonographic findings, the diagnosis was choroidal melanoma in a young patient with ocular melanocytosis. The eye was enucleated. Histopathologic examination revealed ocular melanocytosis with diffuse uveal melanocytosis and amelanotic malignant melanoma of the choroid. The choroidal melanoma apparently arose from a preexisting choroidal nevus. Even young patients with ocular melanocytosis should have regular follow-up examinations for early detection of uveal melanoma.