Abstract : Orthognathic surgery for mandibular prognathism aims to restore functional occlusion and optimize facial aesthetics. Accurate three-dimensional (3D) soft tissue prediction is essential for surgical planning and managing patient expectations. While stereophotogrammetry is considered a gold standard for facial soft tissue capture, portable handheld structured light scanners offer potential advantages in cost, flexibility, and accessibility. However, comparative data on their accuracy for soft tissue prediction in patients with mandibular prognathism remains limited. To compare the accuracy of soft tissue prediction following orthognathic surgery in patients with mandibular prognathism using a portable handheld structured light scanner versus a stationary stereophotogrammetry system. This prospective clinical trial included patients with mandibular prognathism requiring orthognathic surgery. Preoperative soft tissue facial scans were obtained using both a Creality Ferret handheld structured light scanner and a stereophotogrammetry system. Virtual surgical planning was performed using ProPlan CMF software, and soft tissue predictions were generated. Six months postoperatively, follow-up scans were acquired using both modalities. Predicted and postoperative scans were superimposed in 3-matic software using iterative closest point algorithms. Linear deviations were measured at eight standardized facial landmarks: pronasale, subnasale, labrale superioris, labrale inferioris, pogonion, right and left oral commissures, and menton. Statistical analysis included paired t-tests and Wilcoxon signed-rank tests with significance set at p ≤ 0.05. Effect sizes were calculated to determine clinical relevance. Twenty patients (mean age 26.4 ± 5.8 years) completed the study. The handheld scanner demonstrated significantly lower overall mean deviation compared to stereophotogrammetry (2.58 ± 0.82 mm vs. 3.24 ± 1.17 mm; mean difference -0.659 mm; 95% CI: -0.947 to -0.371 mm; p = 0.0029), with a very large effect size (d = -1.58). Landmark-specific analysis revealed significant advantages for the handheld scanner at pronasale (1.57 ± 0.84 mm vs. 1.89 ± 1.00 mm; p = 0.023), labrale inferioris (2.75 ± 1.50 mm vs. 3.17 ± 1.66 mm; p = 0.024), pogonion (1.71 ± 0.38 mm vs. 2.35 ± 1.42 mm; p = 0.028), and both oral commissures (right: 4.80 ± 2.07 mm vs. 5.94 ± 2.19 mm, p < 0.001; left: 2.74 ± 1.30 mm vs. 3.89 ± 1.48 mm, p = 0.009). At the menton landmark, stereophotogrammetry produced consistently non-analyzable surface representations, while the handheld scanner successfully captured all cases (mean deviation 3.00 ± 0.93 mm). No significant difference was observed at labrale superioris (p = 0.520). The portable handheld structured light scanner demonstrated superior accuracy compared to stationary stereophotogrammetry for soft tissue prediction in patients with mandibular prognathism undergoing orthognathic surgery. The handheld scanner showed higher accuracy in the submental area, where stereophotogrammetry exhibited significant limitations. These findings support the clinical applicability of handheld scanning technology as an accurate alternative for 3D facial assessment in orthognathic surgical planning.