Purpose <p>Studies assessing the anterior tibiofibular gap (ATFG) using ultrasonography (US) under various weight-bearing conditions and ankle angles in young athletes are lacking. This study aimed to examine the within-session repeatability of US-measured ATFG under different weight-bearing conditions and ankle angles in young athletes.</p> Methods <p>A total of 49 young athletes aged 9–17 years (32 males, 17 females; 98 ankles) participated in this study. ATFG was assessed at the level of the anterior inferior tibiofibular ligament using US in both sitting and standing postures. The ankle was positioned under three conditions: neutral (0°), 20° dorsiflexion, and 20° dorsiflexion with 30° external rotation. The within-session repeatability of the US-based ATFG measurements was assessed for each weight-bearing condition and ankle angle according to maturation stage. Additionally, ATFG was compared between maturation stages, and sex-related and side-to-side differences were analyzed.</p> Results <p>The within-session repeatability was excellent for all measurement conditions, with intraclass correlation coefficient values exceeding 0.90 and standard error of measurement ranging from 0.05 to 0.21 mm. ATFG was greater in more mature athletes. No sex-related or side-to-side differences were observed in the ATFG or in the widening ratio.</p> Conclusion <p>US-based assessment of the ATFG demonstrated excellent within-session repeatability under standardized measurement conditions in young athletes.</p>