Introduction: This study aims to evaluate the effects of the twin-block (TB) appliance, which is used in the treatment of skeletal Class II malocclusion with retrognathic mandible, on skeletal and dental structures using cephalometric analysis and its effects on soft tissue using 3D images obtained through a topographic scanner (stereophotogrammetry). Methods: In our study, a total of 23 patients who applied to Gülhane Military Medical Academy (GATA), Department of Orthodontics—15 of whom were female and eight male—with Class II skeletal malocclusion with retrognathic mandible, were included. Patients were treated with the TB appliance for 10 months. Changes in dental and skeletal structures were analyzed via cephalometric X-rays, pre-treatment (T0), and post-treatment (T1). Soft tissues were evaluated using 3D images obtained through stereophotogrammetry equipment. Data comparison was performed using the Wilcoxon Signed Ranks test. Results: In the evaluation of cephalometric analyses, 0.5° reduction in the SNA angle (P<0.001), 2° increase in the SNB angle (P<0.001), 3° reduction in the ANB angle (P<0.001), 6 mm reduction in the Wits value (P<0.001), 0.3 mm reduction in the Nv-A distance (P<0.001), and 4 mm reduction in the Nv-Pog distance (P<0.001) were determined. In the measurements obtained with 3D images through stereophotogrammetry, for sagittal dimension soft tissue parameters, 3.6 mm at pogonion (Pog) (P<0.001), 3.98 mm at sulcus inferior (Si) (P<0.001), 2.1 mm at labiale inferior (Li) (P<0.001), 1.48 mm at labiale superior (P<0.039), 1.17 mm at subnasale (Sn) (P<0.001), 1.03 mm at pronasale (Prn) (P<0.001), and 2.36 mm of forward movement at nasion (N) (P<0.001) were observed. When the vertical dimension soft tissue parameters were evaluated, an increase of 4.01 mm in the lower anterior facial height (P<0.001), 1.3 mm in the upper anterior facial height (P<0.001), and 4.3 mm in the total anterior facial height was observed. When dental parameters were examined, a reduction of 5 mm (P<0.001) and 2.5 mm (P<0.001) was determined in overjet and overbite measurements, respectively. Conclusion: The TB appliance, by accelerating the mandible's forward and downward growth, proved to be an effective tool in correcting Class II skeletal malocclusion with a retrognathic mandible. The positive changes in soft tissues, accurately measured using 3D images obtained through stereophotogrammetry equipment, not only contribute to our understanding of orthodontic treatments but also inspire further research and innovation in the field.