Tenosynovial giant cell tumor (TGCT) is a systematically benign but locally aggressive lesion arising from the synovium, tendon sheath or joint bursae. Even in athletes, soft tissue tumors may be the underlying reason or a component of posterior ankle impingement, although the most common mechanism is forceful and repetitive plantar flexion. In this article, we present a case of localized TGCT in a 15-year-old female patient presenting with symptoms of posterior ankle impingement. The preferred technique for treatment was complete local resection via posterior ankle endoscopy. The patient returned to sports at three months and no recurrence was observed on the last follow-up at the first postoperative year. Although rare, soft tissue tumors should be taken into consideration in posterior ankle impingement in athletes. Such benign soft tissue tumors in the posterior ankle can be treated safely and effectively via two-portal posterior endoscopic approach.