Background: The cause of navicular tuberosity pain in adolescents with flexible flatfeet is not well understood. We hypothesized that some of the navicular tuberosity pain may be related to insertional enthesopathy of the posterior tibial tendon at the navicular. Magnetic resonance imaging was performed to look for abnormal signal changes in a series of patients. Material and Method: Consecutive adolescent patients presenting with flexible flatfeet and navicular tuberosity pain were prospectively recruited. A detailed foot examination and body fat analysis was performed. Standing radiographs and Tekscan pedobarograph of both feet were obtained. Magnetic resonance examinations were performed on a 1.5-T whole-body magnetic resonance imaging system utilizing a standard extremity coil. Results: MRI abnormality was detected in 15 of the 36 feet in 18 adolescents examined. Abnormalities detected included thickening of the posterior tibial tendon insertion, marrow edema in the accessory navicular, marrow edema in the navicular tuberosity, and contrast enhancement at the posterior tibial tendon insertion site. Patients with MRI abnormalities were significantly taller, had a lower body mass index and a lower body fat percentage than those without MRI abnormality. Forward stepwise logistic regression analysis identified low body fat percentage and presence of an accessory navicular as independent predictors for abnormality on MRI. Conclusion: MRI abnormality was frequently detected in adolescents with painful flexible flatfeet. The MRI signal changes indicated an enthesopathy like process occurring at the posterior tibial tendon insertion to the navicular which could explain the origin of pain in flexible flatfeet patients without an accessory navicular. Early identification and appropriate treatment to prevent progression may be helpful. Level of Evidence: IV, Case Series