Introduction: Obstructive sleep apnea syndrome (OSAS) is a common clinical disease that seriously affects the quality of life and health of patients. This study aimed to explore the correlation between OSAS and cognitive dysfunction in elderly patients with hypertension. Methods: A total of 106 elderly hypertensive patients were included and divided into OSAS group (n = 45) and non-OSAS group (n = 61), according to whether they combined with OSAS. OSAS was monitored with a portable polysomnography monitor by monitoring sleep and breathing. The cognitive dysfunction of the patients was evaluated using the Montreal Cognitive Assessment (MoCA). Basic data and MoCA scores of the patients were compared between the OSAS group and non-OSAS group. The correlation between OSAS and cognitive dysfunction in patients was evaluated using Pearson’s correlation analysis. Results: The proportion of men (38 vs. 22, p = 0.000), atrial fibrillation (31 vs. 19, p = 0.000), body mass index (27.32 \pm 3.85 vs. 21.27 \pm 5.90, p = 0.002), systolic pressure (167.76 \pm 14.31 vs. 153.22 \pm 12.79, p = 0.008), homocysteine (29.71 \pm 6.27 vs. 12.50 \pm 4.19, p = 0.005), cognitive dysfunction (15 vs. 10, p = 0.042) in patients of the OSAS group were significantly higher compared to patients of the non-OSAS group. Visual space/executive ability (3.12 \pm 1.23 vs. 4.75 \pm 1.03, p = 0.021), memory (2.48 \pm 0.31 vs. 3.71 \pm 0.42, p = 0.039), attention (4.15 \pm 1.21 vs. 5.12 \pm 1.87, p = 0.041), total MoCA scores (20.11 \pm 5.09 vs. 25.76 \pm 4.31, p = 0.017) in patients in the OSAS group were significantly lower compared to patients in the non-OSAS group. OSAS was positively correlated with cognitive dysfunction in elderly patients with hypertension (r = 0.224, p 0.05). Conclusions: OSAS was positively correlated with cognitive dysfunction in elderly patients with hypertension, so OSAS could increase the risk of cognitive dysfunction through its own adverse effects or its accompanying disease status.