Rupture of the tibialis posterior tendon associated with ankle fracture is rare and difficult to diagnose. This rupture can be easily overlooked because the clinical examination is limited owing to acute pain related to a closed ankle fracture. Complete rupture of the tibialis posterior tendon can be identified by a loss of tension during ankle fracture fixation, but partial rupture is more difficult to detect because the tibialis posterior tendon can maintain its tension. A few cases of complete rupture of the tibialis posterior tendon combined with ankle fracture have been reported. It is well-known that failure to diagnose a rupture of the tibialis posterior tendon can lead to long-term disability and a planovalgus foot. However, to our knowledge, this is the first report of partial rupture of the tibialis posterior tendon in the English literature. Herein, we describe a patient with a neglected partial rupture of the tibialis posterior tendon combined with a medial malleolar fracture.