Objective: This study aimed to determine if optic nerve sheath diameter (ONSD) measurement on computed tomography could differentiate transient ischemic attack (TIA) from acute ischemic stroke (AIS). Both TIA and AIS are the rings of the same disease chain. To exclude hemorrhagic stroke and stroke mimics in these patients, brain computed tomography (CT) remains the first step imaging modality.Patients and Methods: In this retrospective study, ONSDs of patients with TIA and AIS within three hours from symptom onset to initial CT was measured. The right, left, mean, and delta ONSD measurements were compared between AIS and TIA groups. Then diagnostic accuracy metrics were calculated.Results: A total of 196 patients (128 in the AIS group and 68 in the TIA group) were included. Both mean and delta ONSD of AIS patients were higher than those of the TIA group. The area under the receiver operating curve of mean and delta ONSD for predicting AIS were 0.746 with a sensitivity of 82.8% and a specificity of 42.7% (cut-off: 5.00 mm), and 0.826 with a sensitivity of 67.2% and a specificity of 86.8% (cut-off: 0.50 mm), respectively.Conclusion: Increased mean or delta ONSD measured on initial CT could alert emergency physicians for an impending stroke.