Objective The complexity of radiographic Tanner-Whitehouse method makes it less acceptable by radiologists and endocrinologists to assess bone age. Conventional ultrasound could be used to measure the ratio of the height of the ossification center to the epiphysis of the bone to evaluate maturity of bone. The purpose of this study is to obtain radiographic TW3 skeletal maturity score with ultrasound images. Methods In this prospective diagnostic study, participants aged between 1 and 18 years undergoing radiography for bone age evaluation were evaluated from April 2019 to November 2021. Ultrasonic skeletal maturity scores of participants were transformed into radiographic skeletal maturity scores with the fitted formulas established in this study. Diagnostic performances of the transformed scores to diagnose advanced or delayed bone age were confirmed. Ultrasound images of 50 participants in the validation group were re-evaluated to confirm inter-rater reliability. Results A total of 442 participants (median age, 9.5 years [interquartile range, 7.8-11.1 years]; 185 boys) were enrolled. Ultrasound determination of bone age had a sensitivity of 97% (34/35, 95% CI: 83, 99) and a specificity of 98% (106/108, 95% CI: 93, 99) to diagnose advanced or delayed bone age. The intra-class correlation coefficient for inter-rater reliability was 0.993 [95% CI: 0.988, 0.996], p < 0.0001. Conclusions Radiographic Tanner-Whitehouse skeletal maturity score could be obtained from ultrasound images in a simple, fast, accurate, and radiation-free manner.