Isolated sphenoid sinus mucocoeles are rare; whereas, benign lesions usually discovered incidentally. These may present with headache, diplopia, visual loss or ocular motor nerve palsies, due to slow expansion and mass effects. Prompt endoscopic removal is the key to prevent permanent sequelae. We present the case of a 63-year femele, who presented with sudden loss of vision of the left eye, along with periorbital and retrobulbar pain for the last one month. She was found to have "no light perception" vision in the left eye and optic atrophy. Neuroimaging was performed, which revealed a sphenoid sinus mucocoele, impinging on the left optic nerve. Our ENT Department performed endoscopic sinus surgery and removed it, but her vision did not return at 6-month follow-up. Although very rare, sphenoid sinus mucocoeles should be considered in the differential diagnosis of any patient, who presents with sudden visual loss. Key Words: Sphenoid sinus, Mucocoele, Optic neuropathy, Visual loss.