Purpose <p>Choroidal alterations are implicated in the pathophysiology of retinal vein occlusion (RVO), yet comprehensive spatial analysis has been limited by conventional imaging. This study aims to leverage ultra-widefield swept-source optical coherence tomography angiography (SS-OCTA) to quantitatively assess spatial changes in the choroidal vascularity index (CVI) and choroidal thickness (CT) in patients with RVO.</p> Methods <p>This cross-sectional study included 38 patients with unilateral RVO and 40 healthy controls. Using ultra-widefield SS-OCTA, volumetric CVI and CT were quantified across a 24 × 20&#xa0;mm area. Data from nine anatomical grids were compared among RVO-affected, contralateral fellow, and healthy control eyes. The correlation between central macular thickness (CMT) and choroidal parameters in RVO eyes was also analyzed.</p> Results <p>Compared to controls, RVO-affected eyes showed significantly decreased CVI in the nasal-superior and optic disc grids, and reduced CT in the nasal-superior, optic disc, and nasal-inferior grids. No significant differences in CVI or CT were found between RVO-affected and fellow eyes. In RVO eyes, CMT was negatively correlated with CVI in the nasal-superior and macular grids, but showed no significant correlation with CT.</p> Conclusion <p>Ultra-widefield SS-OCTA reveals specific spatial patterns of reduced CVI and CT in RVO, predominantly in the nasal and peripapillary regions. These findings suggest choroidal involvement in RVO includes both vascular and stromal components, and that CVI may serve as a sensitive biomarker for monitoring disease-related choroidal alterations.</p>