BackgroundThe emerging role of vitamin D has drawn the attention of researchers around the world, including its involvement in cardiovascular complications among individuals with diabetes. AimThis study aimed to obtain comprehensive evidence on the association between serum vitamin D level and the risk of cardiovascular disease among patients with diabetes. MethodsSystematic search was performed on July 1st, 2023, to identify and screen published literature reporting the association between vitamin D and cardiovascular disease among diabetic patients in six databases. Each eligible study was appraised for its quality using modified Newcastle Ottawa Scale for cross-sectional and cohort studies. Meta-analysis was performed using Dersimonian–Laird random effect model or fix-effect model. The heterogeneity and publication bias were judged based on percentage of I2 and the symmetry of Begg’s funnel plot, respectively. ResultsAs many as 22 studies were found eligible for the systematic review. A meta-analysis from 13 studies comprising of 3850 and 1797 (control and exposure groups, respectively) revealed that serum vitamin D level was significantly lower in patients with diabetes and cardiovascular diseases (Z=4.89; p-total<0.001; SMD=0.68 [95%CI: 0.41–0.95]), yet the heterogeneity was high. Following the adjustment of removing the potential outliers, the same results were still observed (Z=6.19; p-total<0.001; SMD=0.35 [95%CI: 0.24–0.46]). Though decreased, high heterogeneity could not be resolved, resulting in moderate level of this evidence. Another pooled analysis of 7 studies with 4211 patients in control group and 2381 patients in exposure group revealed that lower level of serum vitamin D is a risk factor for cardiovascular disease incidence among diabetic patients (Z=4.89; p-total<0.001; OR: 1.76 [95%CI: 1.4–2.2]). ConclusionSerum vitamin D level status is a risk factor for developing cardiovascular diseases among diabetic patients, hence should be carefully monitored and maintained. PROSPERO registrationCRD42023437698.