Abstract: Vitamin K-dependent Protein S (PS) is a natural anticoagulant acting as a non-enzymatic cofactor for both activated protein C (APC) and tissue factor pathway inhibitor (TFPI). The inventors identify an anti-PS nanobody that very surprisingly enhances the APC-cofactor activity of PS through unknown mechanisms. Very interestingly, this nanobody exerts an antithrombotic effect in injured mesenteric microvessels of mice. As a consequence, it10 constitutes a novel class of antithrombotic agents that could be used for the treatment of acute microthrombosis in pathological states such as sepsis, COVID-19, distal microvascular thrombosis induced by stroke, or sickle-cell disease. Thus, the present invention relates to isolated single-domain antibodies (sdAb) directed against protein S (PS) and polypeptides comprising thereof.