Background: The risk of difficult mask ventilation (DMV) is high in patients with obesity. Therefore, we evaluated the accuracy of ultrasound-measured skin-to-hyoid bone distance (SHD) for predicting DMV in this population. Methods: This prospective observational study included adult patients with obesity scheduled for elective surgery. Preoperative airway assessment included the modified Mallampati test, thyromental distance, sternomental distance, upper lip bite test, mouth opening, neck mobility, STOP-Bang score, and the SHD measured by a handheld ultrasound probe. The mask ventilation grade was evaluated using the 4-level Han score, and grades 3 and 4 were considered as DMV. The primary outcome was the ability of SHD to predict DMV using area under the receiver operating characteristic curve (AUC) analysis. A multivariate model including the STOP-Bang score, modified Mallampati test, upper lip bite test, and SHD was also assessed. Results: Data from 326 patients were analyzed. The DMV incidence was 22/326 (6.7%). Patients with DMV were predominantly male and had higher weight, STOP-Bang score, modified Mallampati grade, upper lip bite class, and SHD than did those with easy mask ventilation. The AUC (95% CI) of the SHD for predicting DMV was 0.88 (0.84–0.92). An SHD > 1.9 cm had a positive-predictive value of 27%, while its negative-predictive value was 99% (for SHD ≤ 1.9 cm). Multivariate analysis revealed that the SHD was an independent predictor of DMV. Conclusions In patients with obesity, SHD measured by a handheld ultrasound probe is an independent predictor of DMV and can accurately predict DMV. An SHD ≤ 1.9 cm can exclude DMV with 99% accuracy.