Pseudoaneurysm resulting from vascular impingement by an osteochondroma is extremely rare. The authors report on the case of a 16-year-old male who had a brachial artery pseudoaneurysm and vessel rupture associated with a humeral osteochondroma. This case suggests that pseudoaneurysm should be considered for the differential diagnosis in patients with soft tissue masses and a cuspidal osteochondroma located near the neurovascular bundle and recommends Doppler sonography or angiography.