The chemotherapeutic drug doxorubicin is cardiotoxic and can cause irreversible heart failure. In addition to being cardiotoxic, doxorubicin also induces activation of coagulation. We determined the effect of thrombin-mediated activation of protease-activated receptor 1 (PAR1) on doxorubicin-induced cardiac injury. Administration of doxorubicin to mice resulted in significantly increased plasma prothrombin fragment 1+2, thrombin-antithrombin complexes and extracellular vesicle tissue factor activity. Importantly, mice expressing low levels of tissue factor, but not factor XII deficient mice, treated with doxorubicin had reduced plasma thrombin-antithrombin complexes compared to controls. To evaluate the role of thrombin mediated activation of PAR1, transgenic mice insensitive to thrombin (Par1R41Q) or to activated protein C (Par1R46Q) were subjected to acute and chronic models of doxorubicin-induced cardiac injury and compared to Par1 wildtype (Par1+/+) and PAR1 deficient (Par1-/-) mice. Critically, Par1R41Q and Par1-/- mice, but not Par1R46Q mice, demonstrated similar reductions in the cardiac injury marker cardiac troponin I, preserved cardiac function and reduced cardiac fibrosis compared to Par1+/+ controls after administration of doxorubicin. Further, inhibition of Gq signaling downstream of PAR1 with the small molecule inhibitor Q94 significantly preserved cardiac function in Par1+/+ but not in Par1R41Q mice subject to the acute model of cardiac injury compared to vehicle controls. In addition, mice with PAR1 deleted in either cardiomyocytes or cardiac fibroblasts demonstrated reduced cardiac injury compared to controls. Taken together, these data suggest that thrombin-mediated activation of PAR1 contributes to doxorubicin induced cardiac injury.