Background Diabetic neuropathy is one of the commonest chronic complications of diabetes seen in routine healthcare and considered the most common cause of peripheral neuropathy all over the world. Vitamin D (VD) deficiency is now recognized as a pandemic disease. This study was designed to explore the levels of 25- hydroxycholecalciferol [25(OH) D] in patients with type 2 diabetes mellitus (T2DM) with peripheral neuropathy. We also aimed to clarify the effect of VD supplementation on cardiometabolic status and electrophysiological pattern of peripheral neuropathy. Patients and methods This clinical trial enrolled 95 patients with T2DM with peripheral neuropathy. The enrolled patients were divided into three groups according to serum 25(OH) D levels. VD deficiency and insufficiency groups received VD supplements (42,000 IU oral VD per week and 500-mg calcium carbonate per day for 12 weeks). Clinical, electrophysiological pattern, and laboratory parameters were evaluated at baseline and after 12 weeks of intervention. Serum 25(OH) D levels were measured by using a competitive enzyme-linked immunosorbent assay kit. Results Our results revealed that, among 95 patients with T2DM with peripheral neuropathy, 32 patients had VD insufficiency [20 ng/ml <25(OH) D <30 ng/ml], 50 patients had VD deficiency [25(OH) D ? 20 ng/ml], and 13 patients had VD sufficiency [25(OH) D >30 ng/ml]. Our results reported that 25(OH) D levels were negatively correlated with cardiometabolic risk factors and Toronto Clinical Scoring System. On the contrary, 25(OH) D levels were positively correlated with nerve conduction velocities (NCV). Stepwise multiple linear regression analysis revealed that glycated hemoglobin and Toronto Clinical Scoring System were the main predictors of 25(OH) D levels among other clinical and laboratory biomarkers. Logistic regression analysis observed that motor NCV and sensory NCV of median nerve and glycated hemoglobin were independent predictors of response to VD supplementation. NCV in studied groups showed that motor NCV and sensory NCV in the median, posterior tibial, and ulnar nerves were significantly decreased in both VD deficiency and insufficiency groups compared with VD sufficiency groups, and supplementation with 42 000 IU oral VD per week and 500-mg calcium carbonate per day for 12 weeks improved cardiometabolic risk factors and electrophysiological pattern of peripheral neuropathy. Conclusion The supplementation of VD for 12 weeks to VD deficiency and insufficiency groups improved the