Background: Giant cell tumor of soft tissue (GCT-ST), which histologically resembles GCT of bone, is a rare tumor. Usually, it is located in the lower extremities and trunk. GCT-ST, occurring in mediastinum, is extremely rare. Case presentation: We encountered an 18-year-old Chinese woman who had mild dull pain on the left side of back. The following chest computed tomography (CT) showed a heterogeneous mass deeply situated in the posterior mediastinum with compression of the lung and invasion of the adjacent rib. On magnetic resonance imaging (MRI), the tumor exhibited predominantly slight hyperintensity on T2-weighted images and intensely heterogeneous enhancement on contrast-enhanced T1-weighted images. The whole body bone scan showed a mildly increased radiotracer uptake in the proximal portion of the left fifth rib, suggestive of local infiltration by the tumor. Surgical resection of the tumor was performed; subsequently, the tumor was histopathologically proved as GCT-ST. Three months after the operation, the patient developed a local recurrence. A brief discussion about the radiological findings, histopathological features, clinical behavior, and a detailed review of the relevant literature are presented. Conclusions: To the best of our knowledge, this is the first case about recurrent primary mediastinal GCT-ST, moreover, this is the first report to introduce the MRI findings of primary mediastinal GCT-ST. The present case highlights the ubiquitous distribution of soft tissue giant cell tumor and the importance of considering this tumor in the differential diagnosis of posterior mediastinal neoplasms. Also, a long-term follow-up is required to properly assess the malignant potential of this tumor.