We present a 4-yr-old boy with adrenocortical carcinoma (ACC), diagnosed due to the appearance of gynecomastia as the presenting symptom. Six months prior to admission, an acute growth spurt along with the development of bilateral breast swelling was observed. He did not present any features of virilization, including enlargement of the testes, increase in testis volume, and penis size. Laboratory investigations showed gonadotropin-independent hypergonadism, with low LH/ FSH levels and elevated estradiol/testosterone levels. Abdominal computed tomography revealed a large heterogeneous mass adjacent to the right kidney and below the liver. Pathological investigations of the biopsy specimen demonstrated that the tumor was an ACC. Pre- and post-operative combination chemotherapy with mitotane was administered and surgical resection was carried out. Post-surgery, the elevated estradiol/testosterone concentrations reverted to within the reference range. Urinary steroid profile and tissue concentration analysis of estradiol and testosterone indicated the presence of estrogen in the ACC tissue. An investigation for TP53 gene aberrations revealed the presence of a germline point mutation in exon 4 (c.215C>G (p.Pro72Arg)). In ACC, the most common symptom is virilization, and feminization, characterized by gynecomastia, is very rare. However, a diagnostic possibility of ACC should be considered when we encounter patients who have developed gynecomastia without the influence of causative factors such as obesity or puberty, and do not present with the typical signs of virilization.