PURPOSE: Subtalar instability (STI) has often been obscured by lateral ankle instability. Moreover, although there have been several reports of techniques for reconstructing STI, no clinical outcome results are known to have been published. The authors report the clinical and radiographic outcomes of the ligament reconstruction of STI with a recently reported novel technique utilising a semitendinosus tendon allograft and interference screws. METHODS: This study is based on 20 ankles that underwent ligament reconstruction for STI between 2009 and 2013. The average follow-up period was 15.0 ± 5 months, and the average age at surgery was 28.1 ± 10.8 years old. Visual analogue (VAS) pain scores, American Orthopedic Foot and Ankle Society (AOFAS) and Karlsson-Peterson ankle scores as well as patient satisfaction were evaluated. Radiographic evaluation of medial translations of calcaneus and subtalar tilt angles was preformed with ankle and Broden's stress radiographs. RESULTS: The VAS pain score decreased from 6.1 ± 1.1 preoperatively to 1.8 ± 1.2 post-operatively (p < 0.05). The AOFAS score improved from 66.0 ± 12.2 preoperatively to 89.6 ± 6.7 post-operatively, and the Karlsson-Peterson score improved from 57.0 ± 13.5 to 91.1 ± 6.8 (p < 0.05). There were no complications such as recurred STI or subtalar joint stiffness. All of the patients were satisfied with the surgery. Subtalar tilt angle decreased from 11.5° preoperatively to 3.0° post-operatively, and the calcaneal medial translations decreased from 7.4 to 3.9 mm. CONCLUSION: This is the first report on the comprehensive clinical and radiographic outcomes of STI reconstruction using a semitendinosus tendon allograft and interference screws. The novel technique of STI reconstruction was found to show encouraging clinical outcomes with high patient satisfaction. LEVELS OF EVIDENCE: IV.