Lacrimal sac septum is a rare structural abnormality of the lacrimal system. A 15-year-old patient presented with epiphora and a mass inferior to the left medial canthal tendon. Dacryocystography and CT-dacryocystography revealed a dilated lacrimal sac with complete obstruction. A horizontal lacrimal sac septum was found in the mid-sac region during endoscopic dacryocystorhinostomy. Histopathologic examination of the septum revealed chronic inflammation with stratified columnar epithelial cell lining consistent with the lacrimal sac.