This study aimed to evaluate the incidence, clinical diagnosis, surgical treatment, and histopathological findings of adnexal masses in children and adolescents. This retrospective study included patients aged < 20 years who were diagnosed with adnexal masses between January 2005 and December 2018 at the Konkuk University Medical Center. Adnexal masses were diagnosed in 406 patients. The mean age of patients was 17.3 years at the time of diagnosis. The primary presenting symptoms and signs were abdominal pain (81.4%), mass per abdomen (13.7%), dysmenorrhea (3.4%), incidental finding (2%), and abdominal distention (0.5%). In total, 204 patients underwent surgery for adnexal masses, and 202 patients were observed without surgery. Histopathological examination revealed 110 benign neoplasms, 72 non-neoplastic lesions, 3 ectopic pregnancies, 3 tubo-ovarian abscesses, 7 borderline malignant tumors, and 9 non-epithelial ovarian malignant tumors. Abdominal pain was the most common reason for hospital visits and surgery in adolescents and young women with adnexal masses. The ultrasonographic diagnosis was consistent with the histopathological diagnosis. In recent years, the use of minimally invasive surgery such as laparoscopy and robotic, has increased in young patients with adnexal masses.