Splenosis is defined as an autotransplantation of the splenic tissue after splenic rupture or splenectomy, and occurs most frequently in the peritoneal cavity. Splenosis is usually asymptomatic and is found incidentally. We report a case of combined intrathoracic and intraperitoneal splenosis in a 54-year-old male who worked as a miner for 10 years in his twenties, and was a current smoker. He was referred to our hospital for further evaluation of an incidental left diaphragmatic mass. Positron emission tomography-computed tomography and bronchoscopy were performed to evaluate the possibility of malignancy. There was no evidence of malignancy, but the spleen was not visualized. Reviewing his medical history revealed previous splenectomy, following a dynamite explosion injury. Therefore, splenosis was suspected and technetium-99m-labeled heat-damaged red blood cell scan confirmed the diagnosis. Radionuclide imaging is a useful diagnostic tool for splenosis, which could avoid unnecessary invasive procedures.