This report details a case of post-traumatic pseudocyst in the spleen that was successfully treated with sclerotherapy using ethanol. A sixteen-year-old boy visited our hospital for a follow-up examination of a splenic cyst. He had experienced blunt trauma to the abdomen three years prior to presentation. An abdominal computed tomography scan revealed a large cyst of the lower pole of the spleen. The cyst was 6.8×9.5×7.0 cm and conservative management was tried. A follow-up ultrasonographic examination three years later revealed that the size of the cyst was unchanged and another treatment was needed to prevent complications. One session of sclerosis with ethanol (90 mL of 99% ethanol) percutaneously was applied to the cyst. A follow-up after four months revealed that the cyst had completely resolved.