IgG4-related disease (IgG4-RD) is a multi-organ immune-mediated chronic fibroinflammatory condition, with unclear certain etiology. It is morphologically characterized by storiform fibrosis, dense IgG4-positive lympho-plasmacytic infiltrate, and obliterative phlebitis. It was recognized as a systemic condition as recently as 2003. IgG4-RD has been described in virtually every organ, forming sclerosing masses, and often mimicking tumors. Clinically, patients present unspecific symptoms and this condition is often recognized incidentally. The epidemiology remains poorly studied, but it has been noted that in the majority of recorded instances, patients are middle-aged men. IgG4-RD could mimic conditions other than tumors, such as infection, inflammation, or other systemic disorders. To ensure accuracy of diagnosis, an exhaustive histopathological analysis is required, together with clinical, radiological, and serological data. Thymic fibrosis in the absence of other primary thymic lesions is a very rare occurrence; in English literature only 1 case has been reported with scattered IgG4 plasma cells infiltrate and focal obliterative phlebitis. We will describe, for the first time, the case of a 49-year-old man displaying an anterior mediastinic, hilar, and intramyocardial mass simulating a sarcoidosis, with a definitive diagnosis of IgG4-related thymic fibrosis extending to the mediastinum and the heart. At the histological examination, we found many features of IgG4-RD in the thymic tissue, such as diffused storiform fibrosis, dense lymphoplasmacytic infiltrate with abundant plasma cells IgG4 positive (ratio IgG/IgG4: 40%), obliterative phlebitis, eosinophilic infiltrate, and Castleman-like lymphoid follicles. We discussed the differential diagnosis and reviewed the literature and the other cases of IgG4-related diseases that had been diagnosed in our department.