Post-traumatic fibro-osseous lesions (PTFOL) are a rare and benign tumor that typically affects the ribs and is probably caused by an excessive post-traumatic reactive process. PTFOL primarily affects the sixth, seventh, and eighth ribs. Here, we report a case of a PTFOL with an unusual location and expansion that simulated a malignant chest tumor. A 28-year-old male patient with a history of minor chest trauma presented with pain. Chest radiography revealed a large, well-defined lesion on the left fourth rib, and computed tomography (CT) of the chest revealed a lytic lesion-type IC on the posterior and middle arches of the left fourth rib with a cartilaginous matrix and discontinued periosteal reaction without soft tissue mass extension. Additionally, magnetic resonance imaging of the chest revealed an ovoid, expansive mass with cystic lobules and lobulated contours extending almost over the entire left fourth rib, measuring 134×47 mm in size. This mass has a low signal on T1-weighted images and a heterogeneous intermediate signal on T2-weighted images, with intense enhancement after gadolinium injection suggestive of a malignant chest tumor. A CT-guided bone biopsy confirmed the presence of an intramedullary lesion consisting of fibrous connective tissue with fusiform fibroblastic cells without atypical signs. The lesion was delimited by bone trabeculae with nibbled edges, indicating exaggerated osteoclastic activity compatible with a diagnosis of PTFOL. The patient was treated with simple analgesics, and chest pain was relieved, with an unchanged volume of the lesion at 1 year of follow-up.