This case reports on a knee locked in flexion due to incarceration of the semitendinosus tendon around an osteochondroma in a patient with a history of multiple hereditary exostoses (MHE). An 18-year-old female with history of MHE presented with acute right medial knee pain and inability to extend her knee. Radiographs confirmed multiple lower extremity osteochondromas, notably a large, 3-cm pedunculated osteochondroma about her right medial proximal tibia. This was assessed as a locked knee secondary to incarcerated hamstring tendons around an osteochondroma. Excision of the osteochondroma restored normal flexion and extension of her knee. At the 3-month follow-up visit, she had returned to all activities with no recurrent medial knee pain or locking. The differential diagnosis for a locked knee joint can be broad, but tendon incarceration should be considered in appropriate patients with significant symptoms. In patients with a history of MHE, osteochondromas may be the cause of tendon entrapment.