Solitary extramedullary plasmacytomas are isolated plasma cell tumors of soft tissue that typically do not metastasize. They are rare and account for 4% of all plasma cell tumors. To our knowledge, only 14 cases of solitary extramedullary plasmacytomas in the sphenoid sinus have been reported. A 32-year-old man presented to our department with complaint of ocular pain in the right eyeball and diplopia. Physical and neurological examinations revealed intact and prompt direct and indirect light reflexes in both pupils and limitation of extraocular muscle movement seen with the lateral gaze of the right eyeball. Magnetic resonance imaging suggested the presence of mucocele or mycetoma, therefore surgical resection was performed with endoscopic endonasal transsphenoidal approach. Histopathology was consistent with plasmacytoma. Systemic work-up did not show any evidence of metastasis and the sphenoid sinus was the sole tumor site, and therefore the diagnosis of solitary extramedullary plasmacytoma was confirmed. We report a rare case of solitary extramedullary plasmacytoma in the sphenoid sinus with successful treatment using the endoscopic endonasal transsphenoidal resection and adjuvant radiotherapy.