The relationships between changes in choroidal vasculature and the severity of diabetic retinopathy (DR) remain unclear. We assessed choroidal changes in diabetic patients by measuring choroidal vascularity index (CVI) in conjunction with DR stage. In this study, patients with diabetes and healthy controls were retrospectively analyzed. Subjects were divided into seven groups as follows: Healthy controls, no DR, mild/moderate non-proliferative DR (NPDR), severe NPDR, proliferative DR (PDR), panretinal photocoagulation-treated DR, and clinically significant macular edema. The mean CVI values in the above groups were 69.08, 67.07, 66.28, 66.20, 63.48, 65.38, and 66.28, respectively. The eyes of diabetic patients exhibited a significantly lower CVI value than those of healthy controls even without DR. The PDR group exhibited a significantly lower CVI value than the healthy control, no DR, and mild/moderate NPDR groups. Age, sex, disease duration, glycated hemoglobin, fasting blood sugar, or intraocular pressure had no correlation with CVI. In multivariate regression analysis, thicker subfoveal choroid and thinner central retina were significantly associated with higher CVI values. These findings carefully suggest that changes in choroidal vasculature could be the primary event in diabetes even where there is no DR.