Abstract Background Optic nerve sheath diameter (ONSD) is a promising noninvasive parameter for intracranial pressure (ICP) assessment. However, in the setting of aneurysmal subarachnoid hemorrhage (aSAH), several previous studies have reported no association between ultrasonically measured ONSD and ICP. In this study, we evaluate ONSD in patients with aSAH using a novel method of automated real-time ultrasonographic measurements and explore whether factors such as having undergone surgery affects its association to ICP. Methods We prospectively included adult patients with aSAH undergoing invasive ICP monitoring. ONSD was obtained using a prototype ultrasound machine with software for real-time automated measurements at the bedside. Correlation between ONSD and ICP was explored, and the ability of ONSD to discriminate dichotomized ICP was evaluated. Abovementioned analyses were performed for the whole cohort and repeated for subgroups by whether the basal cisterns had been surgically entered before ultrasound examination. Results Twenty-six ultrasound examinations were performed in 20 patients. There was a positive correlation between ONSD and ICP (R = 0.43; p = 0.03). In the subgroup where the basal cisterns had not been surgically entered before ultrasound examination, there was a stronger correlation (R = 0.55; p = 0.01), whereas no correlation was seen in the subgroup where the basal cisterns had been surgically entered (R = − 0.16; p = 0.70). ONSD displayed an ability to discriminate ICP dichotomized at ≥ 15 mm Hg (area under the curve [AUC] = 0.84, 95% confidence interval [CI] 0.65–0.96). Subgroup analysis revealed a perfect discriminatory ability (AUC = 1, 95% CI 0.81–1) where the basal cisterns had not been surgically entered and no discriminatory ability (AUC = 0.47, 95% CI 0.16–0.84) where the basal cisterns had been surgically entered before ultrasound examination. Conclusions Automatically measured ONSD correlated well with ICP and displayed a perfect discriminatory ability in patients with aSAH in whom the basal cisterns had not been entered surgically before ultrasound examination, and may be a clinically valuable noninvasive marker of ICP in these patients. Caution should be exercised in using ONSD in patients in whom the basal cisterns have been entered surgically before ONSD measurements, as no association was observed in this subgroup.