Introduction/Aim: Flexible flatfoot (FFF) is one of the most common skeletal deformities in the pediatric population, especially in preadolescence. This retrospective observational study evaluates results of a calcaneal subtalar extra-articular arthrodesis (subtalar arthroereisis) with one cannulated screw for the treatment of painful flexible flatfoot in children, after previously failed conservative treatment. Material and Methods: Preadolescent and adolescent pediatric patients were observed at the Department of Pediatric Orthopedic Surgery. The study included 28 feet of 15 children (5 girls and 10 boys) during a 6-year period, from 2016 to 2021 (average 36 months). The average age was 11.46 years (range 8-15 years). We performed the same operative technique under fluoroscopy for all patients. Radiographic parameters were taken before, after operative treatment, and after hardware removal. The following radiological parameters were measured: talo-calcanear (TC), talo-navicular (TN), talo-first metatarsal bone (T1MT) and calcanear pitch (CP) angles, in the anteroposterior and/or lateral views. Functional outcomes (extension and eversion of the foot) before surgery and after hardware removal were compared. Results: After operative treatment, the midfoot radiographic parameters were not statistically significantly altered; however, the subtalar joint and forefoot were statistically significantly improved: both radiologically and clinically. The range of extension and eversion of the foot were remarkably reduced without disturbing the morphological and functional aspects of the talocrural and subtalar joints. Conclusion: Extra-articular arthrodesis, using a cannulated screw is a minimally invasive technique, represents an optimal method for the operative treatment of symptomatic flexible flatfoot in children. This technique is simple, efficient and does not disrupt the anatomy of the foot.