Orbital leiomyoma is a benign tumour of smooth muscle origin that can be clinically, radiologically and histopathologically difficult to diagnose due to the rarity of leiomyomas in this location. A 74-year-old woman presented with a 2-month history of painless proptosis. Computed tomography and magnetic resonance imaging studies revealed a well-circumscribed intraconal lesion separate from the optic nerve, and not eroding adjacent bone. A right lateral orbitotomy via a lid crease incision was performed to remove the firm lobulated lesion. Postoperatively there was an obvious decrease in proptosis. Microscopically the lesion was a well-circumscribed tumour composed of spindle cells. There were no light microscopic features to suggest malignancy, and immunohistochemistry was used to confirm the diagnosis of leiomyoma. Leiomyoma is a tumour that is rarely found in the orbit most likely due to the paucity of smooth muscle in this location. The differential diagnosis is wide, and the role of radiology, histology and immunohistochemistry in the diagnosis of leiomyoma is discussed. Surgical excision in this case excluded malignancy, provided a definitive diagnosis of a rare entity and resulted in marked clinical improvement.