Retrobulbar abscess is a rare, but severe complication of paranasal sinusitis. The clinical presentations are eyelid swelling, erythema, proptosis, conjunctival chemosis, restricted ocular movement, and decreased visual acuity. Diagnostic methods available for evaluating retrobulbar abscess include sinus X-ray, ultrasonography, computed tomography (CT), and bacterial culture. For the treatment of retrobulbar abscess, immediate surgical drainage and systemic antibiotic therapy are needed. Proper diagnosis and treatments are necessary for preventing visual loss, cavernous sinus thrombosis, subdural abscess, and other lifethreatening complications. A patient, a 30-year-old man, was admitted to our hospital because of progressive eyelid swelling, erythema, ptosis and decreased visual acuity on the right eye after endodontic treatment. The sinusitis occurred secondary to the infection from an upper molar tooth. The spread of the infection led to the orbit via ethmoidal sinus and posterior orbital wall. Immediate surgical intervention was performed and systemic antibiotics was administrated. The symptoms and signs are improved after treatments, so we present our case with a brief review of the literature.