Rationale:Plasmacytoma is a disease caused by the proliferation of monoclonal immunoglobulin-producing plasma cells and divided into multiple myeloma, solitary bone plasmacytoma, and extramedullary plasmacytoma. We report a case of orbital extramedullary plasmacytoma invading the dura mater in a patient with exophthalmos and diplopia.Patient concerns:A 35-year-old female patient with exophthalmos in the right eye and diplopia visited the clinic.Diagnoses:The thyroid function tests showed nonspecific results. Orbital computed tomography and magnetic resonance imaging revealed a homogeneously enhancing orbital mass infiltrating the right maxillary sinus and adjacent brain tissue in the middle cranial fossa through the superior orbital fissure.Interventions:To diagnose and alleviate the symptoms, an excisional biopsy was performed, which revealed a plasmacytoma.Outcomes:One month after the surgery, the protruding symptoms and eye movement restriction in the right eye improved, and the visual acuity in the right eye was restored.Lessons:In this case report, we present a case of an extramedullary plasmacytoma that originated in the inferior wall of the orbit and invaded the cranial cavity. To our knowledge, no previous reports have described a solitary plasmacytoma that originated in the orbit, causing exophthalmos and invading the cranial cavity at the same time.