Background: The aim of this prospective longitudinal study was to evaluate three-dimensional facial soft tissue changes following treatment with the Sander Bite Jumping Appliance (BJA) in growing patients with skeletal class II malocclusion. Methods: Seventeen consecutive growing patients (14 males and 3 females; mean age of 11.7 ± 1.1 years) with skeletal class II malocclusion due to mandibular retrusion were treated with the Sander BJA. Three-dimensional facial scans were acquired before treatment (T0) and after completion of active treatment (T1) using a 3dMD stereophotogrammetric system. Twelve angular soft tissue variables were measured after image superimposition using stable facial reference regions. Changes between T0 and T1 were analysed using paired Student’s t-tests (p < 0.05). Results: The facial convexity angle increased significantly from 155.49 ± 4.12° at T0 to 157.21 ± 3.84° at T1, corresponding to a mean change of 1.72° (95% CI: 0.06–3.39; p = 0.044) and a moderate effect size (Cohen’s dz = 0.53). Among the secondary outcomes, significant changes were observed for the SS-N-SL angle (mean change: −1.29°; p = 0.002; dz = −0.89), upper lip position (−1.09°; p = 0.006; dz = −0.77), LS-N-LI angle (−0.69°; p = 0.042; dz = −0.54), and submental angle (+13.86°; p < 0.001; dz = 1.10). No significant changes were observed for the remaining angular measurements. Conclusions: Treatment with the Sander BJA was associated with measurable three-dimensional soft tissue changes predominantly involving the lower facial region. The observed modifications included mainly profile straightening and changes in lip position. Because of the absence of an untreated control group, these findings should be interpreted as treatment-associated soft tissue changes in growing patients.