This case report describes a 37-year-old woman, who was diagnosed incidentally with thoracic splenosis (TS) in the evaluation of pneumonia, 20 years after a traffic acccident and splenectomy. A ^99mTc sulfur colloid scan demonstrates a multiple area of increased uptake at the left hemithorax and left abdominal area corresponding to the nodules on the CT scans. Splenosis is the heterotopic autotransplantation of splenic tissue after splenic trauma or surgery in the abdominal, pelvic or thoracic cavity. Most TS cases are asymptomatic and are diagnosed incidentally as pleural nodules or masses on a chest radiograph or unrelated diagnostic imaging. Multiple, asymptomatic, left-sided pleura-based lesions associated with a history of splenic trauma or surgery are the key features suggesting a diagnosis of TS. This diagnosis can often be confirmed by superparamagnetic iron oxide (SPIO) -enhanced MRI and radionuclide scintigraphy, noninvasively. Awareness of this diagnosis can avoid unnecessary invasive diagnostic procedures, including surgical intervention or thoracotomy. In conclusion, thoracic splenosis should be considered in a differential diagnosis when patients present with intrathoracic masses accompanied by a history of splenic injury. We should avoid unnecessary invasive diagnostic procedures by a proper imaging diagnosis.