Pseudoaneurysm is extremely rare complication after ankle arthroscopy with standard anteromedial and anterolateral portals. We report a case of a pseudoaneurysm of the anterior tibial artery detected at 3 months after ankle arthroscopy in a 16-year-old male. He had sustained painful swelling of his right ankle after the arthroscopic surgery, and referred to our hospital with an MRI checked postoperatively. We failed to make the diagnosis of pseudoaneurysm with the postoperative MRI, thus the patient underwent another arthroscopy which revealed massive hemarthrosis within the joint. The diagnosis was confirmed with an angiography, and the vascular lesion was ligated.