Background/Aims: Rosai-Dorfman disease (RDD) is a rare, self-limited disorder of unknown etiology that affects children and young adults worldwide and typically manifests as chronic, painless cervical lymphadenopathy. Orbital involvement is very rare and may be an isolated extranodal manifestation or associated with concurrent systemic disease. Adjacent bone involvement is most exceptional, and secondary optic neuropathy has never been reported. Methods: This is a case report with review of the literature. Results: We present a 32-year-old man who, over a 3-month period, developed worsening vision, headache, and vertical diplopia. On examination, there was decreased vision with dyschromatopsia, proptosis, and hypotropia of the left eye. CT scan of the orbits revealed a soft tissue mass inseparable from the lacrimal gland with adjacent bone erosion. Histopathologic evaluation revealed a diffuse infiltrate of histiocytes, lymphocytes, plasma cells, and neutrophils with peripolesis and emperipolesis (tunneling of lymphocytes and plasma cells in the histiocytes’ cytoplasm without destruction), consistent with RDD. Resolution of symptoms as well as of the optic neuropathy was achieved with oral corticosteroids. Conclusion: RDD is an important diagnosis that must be considered in the differential diagnosis of an orbital mass.