Background: The Epworth Sleepiness Scale (ESS) questionnaire is widely used for the assessment of daytime sleepiness in patients with sleep apnea-hypopnea syndrome (SAHS). The aim of this study was to develop a Greek version of ESS, bearing in mind that language is a barrier for application in non-English speaking populations. Methods: The forward and backward translation method by bilinguals was applied. The Greek version of ESS (ESSgr) was then administered to 130 healthy age-matched controls and 211 patients with SAHS with various levels of severity. Reproducibility of ESSgr and also the sensitivity after continuous positive airway pressure (CPAP) treatment were tested. Results: Patients' and controls' ESS score was 11.3 ± 5.1 and 5.6 ± 3.2, respectively (p < 0.001). Total score and individual item score were correlated in both groups. ESSgr score was correlated with body mass index and apnea-hypopnea index (p < 0.001) but not with age. Reproducibility was tested in 29 subjects, revealing no significant difference. A significant reduction was revealed (14.1 ± 4.7 vs. 6.4 ± 3.5; p < 0.001) in 37 patients who were evaluated after CPAP treatment. Conclusions: Our data validate the ESS for application in Greek-speaking populations. Despite relevant influences of language and cultural background, ESSgr is a valuable tool for clinical management and research.