Histidine-rich glycoprotein (HRG) circulates in plasma at a concentration of 2μM and binds plasminogen, fibrinogen, and thrombospondin. Despite these interactions, the physiologic role of HRG is unknown. Previous studies have shown that mice and humans deficient in HRG have shortened plasma clotting times. To better understand this phenomenon, we examined the effect of HRG on clotting tests. HRG prolongs the activated partial thromboplastin time in a concentration-dependent fashion but has no effect on tissue factor–induced clotting, localizing its effect to the contact pathway. Plasma immunodepleted of HRG exhibits a shortened activated partial thromboplastin time that is restored to baseline with HRG replenishment. To explore how HRG affects the contact pathway, we examined its binding to factors XII, XIIa, XI, and XIa. HRG binds factor XIIa with high affinity, an interaction that is enhanced in the presence of Zn2+, but does not bind factors XII, XI, or XIa. In addition, HRG inhibits autoactivation of factor XII and factor XIIa–mediated activation of factor XI. These results suggest that, by binding to factor XIIa, HRG modulates the intrinsic pathway of coagulation, particularly in the vicinity of a thrombus where platelet release of HRG and Zn2+ will promote this interaction.