Current antithrombotic drugs have an adverse effect on bleeding, highlighting the need for new molecular targets for developing antithrombotic drugs that minimally affect hemostasis. Here we show that LIMK1-/- mice have defective arterial thrombosis in vivo but do not differ from wild-type mice with respect to bleeding time. LIMK1-/- mice show a selective defect in platelet activation induced through the von Willebrand Factor (VWF) receptor, the glycoprotein Ib-IX-V complex (GPIb-IX), but not by GPIb-IX–independent platelet agonists. In fact, LIMK1-/- platelets show an enhanced reaction to certain GPIb-IX–independent agonists. The defect of LIMK1-/- platelets in GPIb-IX–mediated platelet activation is attributed to a selective inhibition in VWF/GPIb-IX–induced phosphorylation of cytosolic phospholipase A2 (cPLA2) and consequent thromboxane A2 (TXA2) production. Supplementing a TXA2 analog, U46619, corrected the defect of LIMK1-/- platelets in VWF-induced stable platelet adhesion. Although LIMK1-/- platelets also showed reduced actin polymerization after GPIb-IX–mediated platelet aggregation, actin polymerization inhibitors did not reduce TXA2 generation, but rather accelerated platelet aggregation, suggesting that the role of LIMK1 in GPIb-mediated platelet activation is independent of actin polymerization. Thus, LIMK1 plays a novel role in selectively mediating GPIb-IX–dependent TXA2 synthesis and thrombosis and represents a potential target for developing antithrombotic drugs with minimal bleeding side effect.