Fibrosing mediastinitis is an uncommon benign disorder, and its pathogenesis and management remain unclear. Conventional imaging techniques (chest radiographs, CT, MRI) may suggest its diagnosis but are frequently nonspecific, and it frequently mimics a malignant process by presenting as a mediastinal mass without calcifications, encasing, and infiltrating adjacent mediastinal structures, and showing an overall aggressive behavior. The value of FDG PET imaging in this entity remains largely unknown with only a few case reports in the literature, and often, biopsy is necessary for definitive diagnosis. We report a case of biopsy proven fibrosing mediastinitis highlighting the utility of PET in the evaluation and management of the disease.