Here we report successful thoracoscopic-assisted sclerotherapy of a large, prenatally diagnosed microcystic thoracoabdominal lymphatic malformation in a 3-month-old infant born at 38 weeks gestational age. Compression of the inferior vena cava and aortic displacement was demonstrated on imaging. Treatment options include medical management, percutaneous sclerotherapy, or surgical resection. The latter options were complicated by the location of this lesion. Therefore, sclerotherapy under direct visualization via thoracoscopy was performed with near resolution of the lesion after one treatment. This is the first description of treatment of a lymphatic malformation via this approach reported in the literature.