Premature cardiovascular disease and death with a functioning graft are leading causes of death and graft loss respectively in kidney transplant recipients (KTR). Vascular stiffness and calcification are markers of cardiovascular disease that are prevalent in KTR and associated with subclinical vitamin K deficiency. We performed a single-centre, phase II, parallel-group, randomised, double-blind, placebo-controlled trial (ISRCTN22012044) to test whether vitamin K supplementation reduced vascular stiffness (MRI-based aortic distensibility) or calcification (coronary artery calcium score on computed tomography) in KTR over 1 year of treatment. The primary outcome was between-group difference in vascular stiffness (ascending aortic distensibility). KTR were recruited between September 2017 and June 2018, and randomised 1:1 to vitamin K (Menadiol diphosphate 5mg; n=45) or placebo (n=45) thrice-weekly. Baseline demographics, clinical history and immunosuppression regimens were similar between groups. There was no impact of vitamin K on vascular stiffness (treatment effect -0.23 (95% CI -0.75 to 0.29) x10 mmHg ; p=0.377), vascular calcification (treatment effect -141 (95% CI -320 to 38) units; p=0.124), nor any other outcome measure. In this heterogeneous cohort of prevalent KTR, vitamin K supplementation did not reduce vascular stiffness or calcification over 1 year. Improving vascular health in KTR is likely to require a multifaceted approach.This article is protected by copyright. All rights reserved.