Obstructive sleep apnea (OSA) is a sleep disorder with high prevalence and significant health consequences. The aim of this study was to validate the STOP-Bang Questionnaire, a widely used screening tool for OSA besides polysomnography, in Greek patients. A hundred and two ( 102) patients with OSA, aged [mean (SD)] 59.16 (8.53) years, and 102 healthy adults, aged [mean (SD)] 54.67 (9.36) years, were assessed. Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) were calculated for different cut-off values. The Epworth Sleepiness Scale (ESS) was also used to examine predictive validity. Patients with OSA showed higher scores than healthy controls (p < 0.001). The resulting specificity, sensitivity, PPV, and NPV were 98%, 60%, 71%, 97%, (cut-off 3), 96%, 81%, 84%, 95% (cut-off 4), and 86%, 97%, 97%, 88% (cut-off 5), respectively. This study has demonstrated that the Greek version of STOP-Bang can be used in the clinical setting to differentiate patients with OSA from healthy individuals with high accuracy and at low cost.