Observational and experimental data reinforce the concept that vitamin D is associated with the pathogenesis of arterial hypertension. We investigated the effect of a single dose of 100,000 IU of cholecalciferol, in office blood pressure (BP), and 24-h ambulatory blood pressure monitoring (ABPM) in patients with type 2 diabetes mellitus (DM), hypertension, and hypovitaminosis D. Forty-three patients were randomized to a placebo or cholecalciferol group. BP was assessed by office measurements and 24-h ABPM, before and after intervention. At week 8, a greater decrease in median ABPM values was observed in cholecalciferol supplementation than in the placebo group for systolic 24-h (- 7.5 vs. - 1; P = 0.02), systolic daytime (- 7 vs. - 1; P = 0.007), systolic nighttime (- 7.0 vs. 3; P = 0.009), diastolic 24-h (- 3.5 vs. - 1; P = 0.037), and daytime DBP (- 5 vs. 0; P = 0.01). Office DBP was also reduced after vitamin D supplementation. A single dose of vitamin D3 improves BP in patients with type 2 diabetes, hypertension, and vitamin D insufficiency, regardless of vitamin D normalization. Vitamin D supplementation could be a valuable tool to treat patients with type 2 DM, hypertension, and hypovitaminosis D.Trial registration: Clinicaltrials.gov NCT02204527.