An irreducible ankle dislocation is a rare injury. The cause is a dislocation of the distal fibula anteriorly or posteriorly or the insertion of soft tissue, such as the deltoid ligament or posteromedial tendon. The tibialis posterior tendon can be dislocated through distal tibiofibular diastasis and prevent reduction of the ankle joint. The authors experienced anterolateral ankle fracture dislocation with a diastasis of the distal tibiofibular joint, and reduction was impossible because of impingement of the tibialis posterior tendon dislocated anteriorly through the distal tibiofibular diastasis. This paper reports the treatment of this injury.