Matrix metalloproteases (MMPs) play important roles in normal and pathological remodeling processes including atherothrombotic disease, inflammation, angiogenesis, and cancer. MMPs have been viewed as matrix-degrading enzymes, but recent studies have shown that they possess direct signaling capabilities. Platelets harbor several MMPs that modulate hemostatic function and platelet survival; however their mode of action remains unknown. We show that platelet MMP-1 activates protease-activated receptor-1 (PAR1) on the surface of platelets. Exposure of platelets to fibrillar collagen converts the surface-bound proMMP-1 zymogen to active MMP-1, which promotes aggregation through PAR1. Unexpectedly, MMP-1 cleaves PAR1 at a distinct site that strongly activates Rho-GTP pathways, cell shape change and motility, and MAPK signaling. Blockade of MMP1-PAR1 curtails thrombogenesis under arterial flow conditions and inhibits thrombosis in animals. These studies provide a link between matrix-dependent activation of metalloproteases and platelet-G protein signaling and identify MMP1-PAR1 as a potential target for the prevention of arterial thrombosis.