Midline carcinoma with chromosomal rearrangement of the gene nuclear protein in testis (NUT) is termed NUT midline carcinoma (NMC), and is a rare and highly lethal neoplasm. The gene BRD4 is its most common translocation partner, forming a BRD4-NUT fusion oncogene. NMCs typically occur in the midline, most commonly in the head, neck, or mediastinum, as poorly differentiated carcinomas with variable degrees of squamous differentiation. This is a recently characterized tumor, and thus is still largely unfamiliar to radiologists. We report a 10-year-old boy with NMC involving the posterior mediastinum and invading the left pulmonary vein with an extension into the left atrium; these findings are presented with an emphasis on multidetector computed tomography imaging.