Horner syndrome is described in this case report as a rare complication of bacterial sphenoid sinusitis. A patient presented with miosis, ptosis, and ophthalmic nerve palsy with acute sphenoid sinusistis and cavernous sinus thrombosis on MRI. The impairment of sympathetic fiberscan can be explained through the direct septic effects of the sphenoid sinusitis and indirectly through thrombosis of the cavernous sinus at the level of the carotid plexus.