We treated a patient with inflammatory pseudotumor of the mediastinum that exhibited high uptake of fluorodeoxyglucose on positron emission tomography. A 69-year-old male patient was diagnosed with a mass measuring 70 mm in diameter in the anterior mediastinum as revealed by computed tomography. The lesion showed strong uptake of 18-fluorine fluorodeoxyglucose with a maximum standardized uptake value of 10.24 on positron emission tomography, which was suggestive of a thymic malignancy. Complete resection of the mass was achieved, and the postoperative pathological examination confirmed an inflammatory pseudotumor of the mediastinum arising in a perithymic lymph node. Despite its rarity, inflammatory pseudotumor should be taken into consideration when diagnosing a mass lesion with characteristics suggestive of thymic neoplasm on fluorodeoxyglucose positron emission tomography-computed tomography.