Aim: Type 2 diabetes mellitus (T2DM) is one of the most common comorbidities in patients diagnosed with obstructive sleep apnea syndrome (OSAS). The Hemoglobin Glycation Index (HGI) has been introduced as a new index of glycation, instead of HbA1c. We aimed to evaluate the relationship between the disease severity and HGI in non-diabetic patients diagnosed with OSAS, in comparison to diabetic patients. Methods: Our study included 117 patients with OSAS, of whom 66 were non-diabetic while 51 patients had T2DM. Results: A difference was observed between the groups in terms of age (P = .002), HGI (0.347 ± 0.25 vs. 1.380 ± 1.7; P < .0001), predicted HbA1c (P < .0001), HbA1c (P < .0001), fasting blood glucose (P < .0001), eosinophil count (P = .003), and total supine time (P = .044). The intragroup evaluation of groups 1 and 2 showed no significant relationship between HGI and the severity of OSAS, both in the diabetic and the non-diabetic groups (P > .05). The correlation analysis in the DM group showed HGI to be negatively correlated with predicted HbA1c, fasting blood glucose, Hgb, and RBC, but positively correlated with HbA1c, non-REM stage 3, and supine deep sleep time. In the non-DM group, a positive correlation of HGI was found with BMI, HbA1c, total apnea, central apnea, obstructive apnea, total mixed apnea counts, apnea index, total supine sleep time, and supine deep sleep time. However, a negative correlation was found with the non-REM stage 1 (P < .05 for all). Conclusion: HGI is a new glycation index that shows no significant relationship with the severity of the disease or increased AHI, both in the non-diabetic and diabetic OSAS patients.