Ankle fractures are relatively common orthopaedic injuries; however, irreducible ankle fractures with or without dislocations are a rare, unstable type of injury that require surgical treatment. The structures impeding the accurate reduction of ankle fractures may be soft tissues, such as the deltoid ligament, extensor retinaculum, tendons, or bony fragments between the fracture sites. A 47-year-old male patient with irreducible medial malleolus and distal fibula fracture was referred for treatment. Intraoperatively, it was discovered that failed reduction was due to the interposition of the posterior tibial tendon in the syndesmosis. The posterior tibial tendon was pushed posteriorly through the tibiofibular interosseous interval and relocated to its anatomical position. Reduction and fixation were only achieved after relocation of the tendon. In conclusion, when anatomical reduction is not easily achieved, interposition of the posterior tibial tendon in the syndesmosis should be considered.