Anticoagulant protein S (PS) in platelets (PSplt) resembles plasma PS and is released upon platelet activation; but its role in thrombosis has not been elucidated. Here we report that inactivation of PSplt expression using the Pf4-Cre transgene (Pros1lox/loxPf4-Cre+) in mice promotes thrombus propensity in the vena cava where shear rates are low, but not in the carotid artery where shear rates are high. At low shear rate, PSplt functions as a cofactor for both activated protein C and tissue factor pathway inhibitor, thereby limiting factor X activation and thrombin generation within the growing thrombus and insuring that highly activated platelets and fibrin remain localized at the injury site. In the presence of high thrombin concentrations, clots from Pros1lox/loxPf4-Cre- mice contract but not clots from Pros1lox/loxPf4-Cre+ because of highly dense fibrin networks. Thus, PSplt controls platelet activation as well as coagulation in thrombi in large veins but not in large arteries.