Background: The measurement of the optic nerve sheath diameter (ONSD) has been suggested to be used in the evaluation of intracranial pressure of several etiologies. However, its potential utility in the clinical evaluation of patients with idiopathic intracranial hypertension (IIH) needs to be clarified. Methods: We recruited all the IIH patients who had been admitted to our neurology clinics and had a cranial MRI before lumbar puncture investigation. A control group of patients with migraine was also included. Studies were reviewed blindly by a radiologist, and ONSD and ONSD/ eyeball transverse diameter (ETD) for both eyes were measured. Results: Ultimately, we have enrolled 50 patients with IIH and 53 migraineurs. The right ONSD values were higher in the IIH group (p = 0.024) whereas the values of ONSD/ETD were found to be both higher in the IIH group (right: p = 0.006, left: p = 0.043). The ROC curve demonstrated an area under the curve of 0.620 (95% CI = 0.508 to 0.731) for ONSD, and it was 0.642 for ONSD/ETD. Using a cut-off of 6.3 mm, ONSD had the following performance characteristics: sensitivity 18%, specificity 81%. Conclusions: The ONSD and ONSD/ETD values may be utilizable in the evaluation processes of IIH patients; however, they do not solely reach sufficient discriminative accuracy. The potential significance of these parameters in rather monitoring the IIH patients may constitute a strictly crucial topic of interest for future research.