We report herein a case of prenatally detected neonatal adrenocortical tumor (ACT). The patient was an otherwise healthy newborn girl. No signs of Beckwith-Wiedemann syndrome were identified, and her family medical history did not suggest predisposition to cancer. Computed tomography and ultrasonography after birth revealed a round solid tumor 40 mm in diameter in the right suprarenal area. The precise diagnosis of ACT was unexpectedly obtained based on results from the Japanese neonatal mass screening program. Blood 17-hydroxyprogesterone is routinely measured as a part of this program for early detection of congenital adrenal hyperplasia in Japan. Abnormally elevated level of 17-hydroxyprogesterone was reported in the patient and, thus, led to the diagnosis of ACT. Surgical resection was safely performed with perioperative steroid replacement. Adrenocortical tumors are extremely rare in childhood, particularly in the neonatal period. Some of these tumors secrete abnormally high levels of cortisol, suppressing function of the contralateral adrenal gland and, thus, leading to life-threatening postoperative adrenal insufficiency. Scheduled steroid replacement enables safe perioperative management in such cases. Adrenocortical tumor should always be considered among the differential diagnoses for neonatal suprarenal mass because precise diagnosis will enable the physician to develop appropriate treatment strategies.