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Abstract Background Nonimmune hydrops fetalis (NIHF) can be caused by different types of etiologies. Some rare intrathoracic lesions are associated with NIHF. Combination of mediastinal teratoma, NIHF, and chylothorax is extremely rare. Mediastinal teratomas which are located in midline should be difficult to be detected. Thoracic imaging should be performed with unknown etiology for hydrops, and in case of chylothorax, the presence of a mass compressing the ductus thoracicus should be considered primarily. Case presentation An infant was born with a diagnosis of NIHF. Bilateral chest tubes were inserted cause of bilateral pleural effusions. After enteral feeding, the previously clear pleural fluid became chylous. Medium-chain triglyceride infant formula and somatostatin analog octreotide were initiated. A mass was appeared on her neck with the disappearance of skin edema. Magnetic resonance imaging confirmed a large, heterogeneous mass which was suggesting immature teratoma originating from thyroid gland. Complete surgical excision of the mass was performed. Histology confirmed high-grade immature teratoma. The neonate made an uneventful recovery. Following complete cessation of pleural fluid drainage, octreotide was stopped. She was discharged home on exclusive breast milk on day 34 of life. Conclusions Mediastinal teratomas are rare masses that cause hydrops fetalis. Although the association of NIHF and mediastinal teratoma is rare, thoracic imaging can be performed if an etiology cannot be found despite basic evaluations for hydrops. In case of chylothorax, the presence of a mass compressing the ductus thoracicus should be considered primarily, and thoracic imaging should be performed.