Purpose: Obstructive sleep apnea syndrome (OSAS) patients are at risk of glaucoma but the risk increases if they have higher intraocular pressure (IOP) while sleeping. We aim to evaluate the postural effect of upright and prolong supine positions on IOP in these patients.Methods: This is a cross-sectional study involving 27 patients with symptoms of OSAS seen at a tertiary institutional center and 25 normal controls performed between June 2015 and June 2016. All patients and controls underwent a polysomnography (PSG) test and were diagnosed with OSAS based on the apnea-hypopnea index (AHI). Patients are those with OSAS symptoms and had AHI > 5, whereas controls are staffs from the ophthalmology clinic without clinical criteria for OSAS and had PSG result of AHI p = 0.033) and left eye (LE) (1.20 ± 1.26 vs 0.48 ± 0.62 mmHg, p = 0.044). After 30 min supine, the IOP in OSAS patients showed a further increase in IOP than controls (RE 1.20 ± 1.79 vs 0.12 ± 0.66 mmHg, p = 0.001 and LE 0.59 ± 2.00 vs 0.15 ± 0.78 mmHg, p = 0.246). Higher IOP when sitting post supine is associated with higher IOP on supine position.Conclusion: IOP increase is significantly more in OSAS patients on prolong supine position.