A fourth branchial pouch sinus is a rare clinical entity. The handful of previously reported cases have all involved children. We report a rare case of an intrathyroidal fourth branchial pouch sinus in an adult. A 36-year-old man presented with a recurrent left-sided deep neck abscess. A diagnostic evaluation by barium pharyngoesophagography in conjunction with computed tomography revealed the presence of a sinus tract that extended inferiorly from the left piriform sinus to the left thyroid lobe. The synchronous use of both of these imaging modalities with shared contrast enabled us both to establish the diagnosis and formulate a successful surgical approach. The patient underwent elective neck exploration with resection of the left thyroid lobe and superior sinus tract. Postoperative pathology confirmed the presence of an epithelium-lined sinus within the left lobe of the thyroid consistent with a branchial pouch derivative. Follow-up barium pharyngoesophagography demonstrated successful closure of the sinus tract. This unique case represents a delayed clinical presentation, and it broadens the differential diagnosis of recurrent deep neck abscess and thyroid cyst in the adult.