Mammary duct ectasia is a rare disease in children and often presents as a cystic mass with bloody nipple discharge. The pathophysiology of mammary duct ectasia is unclear, and the differential diagnosis of other cystic masses with hemorrhage, such as complicated lymphangioma, is necessary. Here, we report a 14-month-old boy who exhibited unilateral mammary duct ectasia with bloody nipple discharge that was treated with surgical excision. Because some authors have reported that mammary duct ectasia can be often be resolved without surgery, conservative therapy should be considered first when a child presents with a cystic mass with bloody nipple discharge. However, the optimal duration of follow-up and timing of surgical excision have not yet been established.