Gynecomastia or benign proliferation of the male breast glandular tissue is not uncommon for adolescent males. Its pathogenesis has been attributed to a transient imbalance between estrogens and androgens. Ginseng is a popular herb with a long history of medicinal use. Oriental folk medicine describes it as both a tonic for restoring strength and a panacea. The term "ginseng" generally refers to a plant, Panax ginseng. Based on estrogen-like actions of Panax ginseng due to its structural similarity with estradiol, this agent could be speculated to cause gynecomastia. Here we report a 12-year-old Korean-Japanese boy with bilateral enlargement of the breasts with tenderness in the right breast, which was noticed about 1 month before his first visit to our outpatient clinic. He was diagnosed with gynecomastia based on physical, laboratory, and ultrasound examinations. Detailed questioning about his medications and supplements revealed that he had been given red ginseng extract daily to enhance his performance for 1 month before his clinical presentation. He wanted to make his body stronger as an athlete. He was recommended from his grandmother to take Panax ginseng for his purpose. After stopping this, there was no further growth of the masses and no pain when his right breast was pressed. In conclusion, physicians should consider ginseng in the investigation of gynecomastia.