We report a patient with branch retinal vein occlusion (BRVO) accompanied by superior ophthalmic vein (SOV) thrombosis resulting from severe SOV enlargement in a patient with Graves ophthalmopathy (GO). A 78-year-old woman diagnosed with BRVO presented with reduced vision. She had a more than 20-year history of unilateral GO. Fluorescein angiography and spectralis optical coherence tomography revealed BRVO with cystoid macular edema. Approximately 5 months later, she complained of advanced left proptosis. A computed tomography scan at that time revealed dilated SOV with high internal attenuation due to SOV thrombosis of the left orbit. Because of the uncertain onset of SOV thrombosis and minimal signs of acute orbital congestion, we prescribed prophylactic daily aspirin to improve blood flow rather than a heparin-based treatment. In addition, she was treated with lubricants for exposure keratopathy and a dorzolamide/timolol eyedrops to prevent increases in intraocular pressure. This is the first clinical report of BRVO accompanied by SOV thrombosis due to SOV enlargement in chronic severe GO. It should be recognized that SOV thrombosis can lead to an increase in proptosis even in chronic GO.