BACKGROUND/OBJECTIVES: A reliable assessment of gluten-free diet (GFD) adherence is essential for managing celiac disease (CD). This study aimed to validate the Hellenic version of the Celiac Disease Adherence Test (H-CDAT) to evaluate adherence levels and explore the impact of dietary adherence on health-related quality of life (HRQoL)-both of which have never been objectively assessed in Greek CD patients. METHODS: The study included 102 adult CD patients who completed H-CDAT, diet-related questions, and the 36-Item Short Form Health Survey (SF-36). RESULTS: H-CDAT demonstrated good psychometric properties and showed multiple strong correlations with HRQoL dimensions. H-CDAT adherence was Good in 38.2%, Moderate in 42.2%, and Poor in 19.6% of patients, despite their perception of effective adherence, with 51% never having visited a dietitian for guidance on a GFD. Patients scored significantly lower across most HRQoL dimensions compared to the general Greek population. When stratified into the three adherence categories, striking differences emerged between Good and Moderate adherence across both physical and mental health domains, highlighting that moderate adherence is not sufficient for optimal health outcomes. CONCLUSIONS: These findings emphasize the critical role of strict GFD adherence in improving overall health and underscore the importance of dietetic intervention for achieving optimal patient outcomes.