Background: Invasive intracranial pressure (ICP) can result in complications, pain, or even aggravate intracranial hypotension (IH) or headache in patients with IH. Objective: To investigate whether ultrasonographic measurements of optic nerve sheath diameter (ONSD) could serve as a noninvasive IH marker. Methods: Ultrasonographic ONSD was measured prior to lumbar puncture (LP) and ONSD measurements compared to LP's opening pressure. We analyzed correlations between ONSD and ICP and determined the optimal ONSD cut-off point for IH. According to their LP on admission, patients were divided into three groups: IH group, normal ICP group, and elevated ICP group. Correlations between ONSD and ICP were analyzed using Pearson's correlation. A receiver operating characteristic (ROC) curve was generated to determine the optimal cut-off point for IH. Results: In total, 136 subjects (75 men, 55.1% men) were included, and 1088 ONSDs were measured. The ONSD of the IH group (2.96 +/- 0.15 mm) was significantly lower than that of the normal (3.59 +/- 0.33 mm) and elevated ICP groups (4.90 +/- 0.42 mm, p < 0.001). There was a significant difference in ONSD within all groups (p < 0.001), and the differences between the two groups were also statistically significant. ONSD and ICP values were strongly correlated, with an r = 0.952 (95% confidence interval [CI]: 0.924-0.969; p < 0.001). After adjusting for age, diastolic blood pressure, systolic blood pressure, body mass index, waistline, and head circumference, ONSD was positively associated with ICP. ROC curve analysis revealed an area under the curve of 0.990 (95% CI: 0.975-1.000). The ONSD cut-off point for identifying decreased opening pressure on LP was 3.15 mm, with 98.3% sensitivity and 91.7% specificity. Conclusions: Ultrasonographic ONSD may be a noninvasive, valuable, and easy-to-perform bedside technology for evaluating IH.