Optic neuritis (ON) is often the initial symptom of neuromyelitis optica spectrum disorder (NMOSD) and myelin oligodendrocyte glycoprotein-associated disease (MOGAD). We aimed to compare the frequency and pattern of chiasmatic lesions in MOGAD-related ON (MOGAD-ON) and NMOSD-related ON (NMOSD-ON) using conventional brain imaging (magnetic resonance imaging [MRI]) in Latin America (LATAM). We reviewed the medical records and brain MRI (≤30days from ON onset) of patients with a first event of MOGAD-ON and NMOSD-ON. Patients from Argentina (n=72), Chile (n=21), Ecuador (n=31), Brazil (n=30), Venezuela (n=10) and Mexico (n=82) were included. Antibody status was tested using a cell-based assay. Demographic, clinical, imaging and prognostic (as measured by the Visual Functional System Score [VFSS] of the Expanded Disability Status Scale) data were compared. A total of 246 patients (208NMOSD and 38MOGAD) were included. No differences were found in gender and ethnicity between the groups. We observed chiasmatic lesions in 66/208 (31.7%) NMOSD-ON and in 5/38 (13.1%) MOGAD-ON patients (p=0.01). Of these patients with chiasmatic lesions, 54/66 (81.8%) and 4/5had associated longitudinally extensive optic nerve lesions, 45/66 (68%) and 4/5had bilateral lesions, and 31/66 (47%) and 4/5showed gadolinium-enhancing chiasmatic lesions, respectively. A positive correlation was observed between VFSS and presence of bilateral (r=0,28, p<0.0001), chiasmatic (r=0.27, p=0.0001) and longitudinally extensive lesions (r=0,25, p=0.0009) in the NMOSD-ON group, but no correlations were observed in the MOGAD-ON group. Chiasmatic lesions were significantly more common in NMOSD than in MOGAD during an ON attack in this LATAM cohort. Further studies are needed to assess the generalizability of these results.