Ankle fractures involving the posterior malleolus generally lead to worse outcome. However, no studies on gait in trimalleolar ankle fractures have evaluated the influence of size and comminution of the posterior malleolar fragment. We expected patients with more severely comminuted posterior malleolus, more severe fracture type and larger posterior fragment to have reduced gait kinematics and poorer patient-reported outcomes. 26 trimalleolar ankle fracture patients were compared with 14 healthy controls and kinematically analyzed using the Oxford Foot Model. Functional outcome was based on 4 patient reported outcome questionnaires. Effects of posterior fragment size, comminution and Haraguchi fracture classification were determined on conventional and 3D CT-scans. Trimalleolar patients had lower walking speed and reduced range of motion between the hindfoot and tibia in both loading and push-off phases in the sagittal and transverse planes. The range between the hindfoot and tibia in the sagittal plane in the push-off phase correlated significantly with patient reported outcomes. The absolute and relative surface area of the posterior fragment on conventional CT-scans and 3D CT-scans, correlated significantly with range of motion. Patients with a posterior malleolus size >10% of the posterior malleolus had lower flexion-extension between forefoot and hindfoot during loading phase than patients with a size ≤10%. Trimalleolar fractures reduce walking speed and range of motion in the talocrural joint. Reduced range in the talocrural joint is associated with poorer outcomes. Posterior fragment size correlated significantly with range of motion in talocrural and midfoot joints and with patient reported outcomes. Level of evidence: Level 3, retrospective study. •Malleolus tertius fracture size has impact on the range of motion of the ankle.•Patients with trimalleolar fractures have a lower walking speed.•Trimalleolar ankle fractures have decreased range of motion talocrural.•Decreased range of motion talocrural is related to worse functional outcome.