Background Malignant transformation in mature teratomas, especially somatic-type malignancies, are extremely rare phenomena with few cases described in the literature. In the mediastinum, these lesions are often accompanied by metastases, ultimately unmasking them. Case presentation We report an exceptional case of metastatic adenocarcinoma to the breast arising from a mediastinal mature teratoma with malignant transformation in a 31-year-old female. The patient initially presented with a breast lesion that was later diagnosed as either a triple negative ductal carcinoma or a metastatic adenocarcinoma of unknown origin. Extensive clinical workup and radiological imagining revealed an anterior mediastinal mass. Subsequent pathological examination of the mass showed a mature teratoma with malignant transformation equivalent to that found in the breast biopsy. Conclusion Mediastinal teratomas with malignant transformation (TMTs) are rare entities; however these tumors should be suspected in cases of suspicious mediastinal masses presenting with unusual clinical and radiological signs, such as metastasis of unknown primary origin. Pathological corroboration of extensive sampling and meticulous microscopic examination is crucial for the diagnosis in order to tailor subsequent treatment.