Background: Extramedullary plasmacytoma (EMP) is a rare form of plasma cell neoplasm characterized by monoclonal plasma cell infiltration without systemic evidence of multiple myeloma. It represents 1.6-4% of all plasma cell tumors. The most common sites of involvement are the upper respiratory and gastrointestinal tracts, while anterior chest wall involvement is rare and can mimic other common thoracic malignancies. Here, we report a case of a 50-year-old man presenting with a large anterior chest wall mass diagnosed as extramedullary soft tissue plasmacytoma occurring years after blunt thoracic trauma. Plasmacytoma should be considered in the differential diagnosis of anterior chest wall masses, especially when associated with a history of trauma and presence of invasive features on imaging. A combination of clinical presentation, imaging, and biopsy is critical for diagnosing atypical soft tissue neoplasms. Case Presentation: A 50-year-old man presented with a progressively enlarging anterior chest wall mass, initially noticed 5–6 years after blunt thoracic trauma. The mass remained stable for years but rapidly increased in size over six months, causing dyspnea, dysphagia, and odynophagia. Physical examination revealed an 18 cm, firm, non-mobile mass with superficial venous prominence. The initial differentials included thymoma, teratoma, thyroid neoplasm, and lymphoma. However, the history of trauma, chronic presentation, recent progression, and normal tumor markers raised suspicion for a soft tissue or bony lesion. Contrast-enhanced CT showed a lobulated, calcified mass in the superoanterior mediastinum with scattered enhancement, sternal erosion, invasion of the first rib, anterior chest wall extension, and tracheal compression. Laboratory workup revealed hemoglobin 11.6 g/dL, calcium 9 mg/dL, creatinine 0.8 mg/dL, and normal TSH, LDH, β-hCG, and AFP levels, but urine electrophoresis demonstrated an M spike with significantly elevated free kappa light chains (1956.2 mg/L). Histopathology showed diffuse sheets of plasma cells with eosinophilic cytoplasm and eccentric nuclei. Immunohistochemistry was positive for CD138, MUM1, CD56, and kappa light chain, consistent with a diagnosis of kappa-restricted extramedullary plasmacytoma. Discussion: This case demonstrates a rare post-traumatic extramedullary plasmacytoma with extensive local invasion, highlighting the need to consider plasma cell neoplasms in the differentials of anterior mediastinal masses. In this case, scar tissue from prior injury may have created a permissive microenvironment, allowing delayed plasma cell proliferation. Imaging revealed aggressive features suggestive of soft tissue plasmacytoma, and biopsy confirmed the presence of plasma cells. While radiotherapy is the primary treatment modality, chemotherapy may also be used in combination based on the tumor characteristics.