Increased endothelial cell (EC) proliferation is a hallmark of arteriovenous malformations (AVMs) in hereditary hemorrhagic telangiectasia (HHT). The underlying mechanism and disease relevance of this abnormal cell proliferative state of the ECs remain unknown. Here, we report the identification of a CDK6-driven mechanism of cell cycle progression deregulation directly involved in EC proliferation and HHT vascular pathology. Specifically, HHT mouse liver ECs exhibited defects in their cell cycle control characterized by a G1/S checkpoint bypass and acceleration of cell cycle speed. Phosphorylated retinoblastoma (p-RB1)-a marker of G1/S transition through the restriction point-significantly accumulated in ECs of HHT mouse retinal AVMs and HHT patient skin telangiectasias. Mechanistically, ALK1 loss of function increased the expression of key restriction point mediators, and treatment with palbociclib or ribociclib, two CDK4/6 inhibitors, blocked p-RB1 increase and retinal AVMs in HHT mice. Palbociclib also improved vascular pathology in the brain and slowed down endothelial cell cycle speed and EC proliferation. Specific deletion of Cdk6 in ECs was sufficient to protect HHT mice from AVM pathology. Thus, CDK6-mediated endothelial cell cycle acceleration controls EC proliferation in AVMs and is a central determinant of HHT pathogenesis. We propose that clinically approved CDK4/6 inhibitors have repurposing potential in HHT.