Purpose This study aims to assess choroidal vascularity index (CVI) and choroidal thickness (CT) in patients diagnosed with type 1 diabetes mellitus (DM) and compare them with healthy control subjects. Methods This retrospective and cross-sectional study includes 43 patients diagnosed with type 1 DM and 43 healthy age/gender-matched subjects as the control group. Enhanced depth imaging optical coherence tomography (EDI-OCT; Spectralis, Heidelberg Engineering GmbH, Heidelberg, Germany) images of all participants were analyzed. CT measurements of five different points (subfoveal, 500 µm temporal, 1500 µm temporal, 500 µm nasal, and 1500 µm nasal to the fovea) were obtained. Choroid images were divided into luminal (LA) and stromal areas (SA) determined by image binarization method. Choroidal vascularity index (CVI) was defined as the ratio of LA to total choroid area (TCA). The effects of age, HbA1c, fasting plasma glucose, duration of DM, mean blood pressure, intraocular pressure, and axial length measurements on CVI were investigated. Results There was no significant difference between any of the CT measurements of the groups (all p>0.05). There was no significant difference between the groups according to TCA (0.82±0.24 vs. 0.80±0.19; p=0.065), LA (0.54±0.17 vs. 0.55±0.16; p=0.123) and SA (0.28±0.06 vs. 0.25±0.07; p=0.068). However, patients had significantly lower CVI compared to control subjects (66.07±3.19 vs. 68.89±2.85, p<0.001). Univariate linear regression model revealed that there was a negative correlation between disease duration and CVI (standardized beta coefficient:-0.416, p=0.006). Conclusion According to our findings, reduced CVI may indicate subclinical dysfunction in choroid of patients with type 1 DM.