Teratomas are defined by the presence of cell types from different germ layers, they typically involve the gonads or the sacrococcygeal region and are rarely retroperitoneal. Prenatally detected adrenal teratomas are extremely uncommon. Aim of this paper is to share our experience with an adrenal antenatal mass initially diagnosed as a left adrenal neuroblastoma that turned out to be a mature teratoma after microscopical examination. We present the case of a male fetus with antenatal diagnosis of a left adrenal cystic image at the 22nd week of amenorrhea. The fetal magnetic resonance imaging showed a non-calcified cystic mass of the left adrenal gland, compatible with a neuroblastoma. At birth an ultrasound confirmed the presence of an anechogenic lesion of the left adrenal gland. The infant was closely monitored during his first year and in the absence of significant regression of the adrenal mass, it was decided to perform a laparoscopic left adrenalectomy. Unexpectedly, the final pathological diagnosis was mature cystic adrenal teratoma. In conclusion, an adrenal mass diagnosed antenatally is generally a hemorrhage or a neuroblastoma. Adrenal teratomas are very rare and those diagnosed antenatally even more. At present, we have no clinical, biological, or radiological evidence to suspect them before surgical removal. There are only two other cases of unexpected adrenal teratoma in infants described in Literature.