The present case report demonstrates a rare finding associated with irreducible ankle fracture dislocations. To our knowledge, posterior tibial tendon entrapment with an intact ankle mortise has not yet been documented in published studies. In the case of our patient, a high-energy, 12-ft fall resulted in a comminuted intra-articular fracture of the medial malleolus, confirmed by the initial radiographs. Preoperative magnetic resonance imaging showed the Achilles tendon to be ruptured and the posterior tibial tendon to be both displaced and entrapped between the medial malleolar fracture fragments, preventing initial closed reduction. At operative repair for the ruptured Achilles tendon and the medial malleolus fracture, the posterior tibial tendon was removed from the fracture site and was found to be intact with no evidence of laceration or rupture. The tendon was returned back to its anatomic position, and the tendon sheath was reapproximated. Although uncommon, it is important that entrapment of the posterior tibial tendon be considered in cases of irreducible ankle fracture. This injury type can be addressed during open reduction internal fixation to achieve reduction.