Branchial cleft anomalies are a heterogeneous group of congenital disorders that theoretically emerge due to incomplete obliteration of the branchial apparatus, components of the six main pairs of pharyngeal arches, during embryonic development. They can result in a cyst, a sinus, or a fistula. For a congenital lateral neck mass, they represent the most common diagnosis in pediatric neck pathology. Its location is usually in the cervical area, anterior to the sternocleidomastoid muscle. In adults, they present with symptoms such as acute suppurative thyroiditis and recurrent cervical abscess. In this paper, we report the case of a 54-yearold Hispanic woman with a recent history of a left posterior mediastinal mass, detected on computed tomography (CT) scan while studying her recent onset of asthma. The patient underwent an assisted thoracoscopic excision. During the procedure, the mass appeared to be a very large cyst; on histopathological examination, the cyst was determined to be a branchial cleft cyst. To our knowledge, this is the first reported case of this entity located in the posterior mediastinum in an adult patient. Being an unusual and interesting case, it highlights the idea of considering these anomalies when establishing a differential diagnosis of a posterior mediastinal mass.