Acute anterior ischaemic optic neuropathy (AION) is a feared ischaemic complication of giant cell arteritis (GCA). However, distinguishing arteritic AION (A-AION) from its non-arteritic counterpart (NA-AION), which accounts for approximately 90% of cases, can be challenging. Rapid initiation of glucocorticoids is essential to prevent irreversible vision loss in GCA but is unnecessary in NA-AION. This study evaluated the use of orbital MRI in differentiating A-AION from NA-AION. In this prospective single-centre study, patients >50 years who had recent-onset AION were enrolled between June 2021 and October 2024. The final diagnosis of GCA was confirmed after comprehensive assessments and ≥6 months follow-up. Orbital MRIs were independently evaluated by two experienced radiologists blinded to the clinical diagnosis. Of the 18 patients analysed, nine had A-AION (two with bilateral involvement), while nine had NA-AION. MRI demonstrated differences between the groups, notably in contrast enhancement of the ophthalmic artery (72.7% vs 22.2%; p=0.07), perineural fat (90.9% vs 22.2%; p=0.005) and retrobulbar fat (100% vs 11.1%; p<0.001). The most discriminative MRI feature was retrobulbar fat enhancement, achieving 100% (95% CI 72 to 100) sensitivity and 89% (95% CI 52 to 100) specificity. Bilateral orbital enhancement was identified in more than half of the unaffected contralateral eyes in A-AION patients. These results suggest that orbital MRI may help clinicians rapidly differentiate between A-AION and NA-AION to provide the most appropriate treatment.