AbstractA 43‐year‐old otherwise healthy woman was found to have a retroperitoneal mass during a routine medical examination and was referred for further evaluation. Abdominal CT scan showed a well‐delineated, low‐density area that exhibited heterogeneous contrast enhancement. The area measured about 20 mm in size and was to the left of the aorta at the level of the inferior mesenteric artery. MRI showed a mass with heterogeneous hypointensity on T1‐weighted images and heterogeneous hyperintensity on T2‐weighted images. PET‐CT scan showed slightly increased 18F‐fluorodeoxyglucose accumulation within the mass. Laparoscopic resection was performed. A smooth, brownish mass was seen in the retroperitoneum and was resected with minimal blood loss. Histopathological examination showed a nodular mass measuring 40 × 26 × 20 mm that was composed solely of ectopic thyroid tissue. This case shows the exceptional development of ectopic thyroid in the infradiaphragmatic retroperitoneum and demonstrates the usefulness of laparoscopy for resecting such masses.