Thoracoabdominal duplication cysts are a congenital malformation of the posterior primitive foregut with synchronous thoracic and abdominal duplication cysts being found in up to 15% of cases. The presentation of duplication cysts depends on their location, size, and other factors, like the presence of ectopic functioning gastric mucosa, which is reported in 20%–30% of duplication cysts. 99mTc-pertechnetate scintigraphy along with SPECT/CT helps in the preoperative localization of ectopic functioning gastric mucosa in these duplication cysts. We report the scintigraphic and SPECT/CT findings of one such case, which helped in the final diagnosis and management of the patient.