P latelets are central players in atherothrombosis development in coronary artery disease. The PKC family provides important intracellular mechanisms for regulating platelet activity, and platelets express several members of this family, including the classical isoforms PKCα and PKCβ and novel isoforms PKCδ and PKCθ. Here, we used a genetic approach to definitively demonstrate the role played by PKCα in regulating thrombus formation and platelet function. Thrombus formation in vivo was attenuated in Prkca^–/– mice, and PKCα was required for thrombus formation in vitro, although this PKC isoform did not regulate platelet adhesion to collagen. The ablation of in vitro thrombus formation in Prkca^–/– platelets was rescued by the addition of ADP, consistent with the key mechanistic finding that dense-granule biogenesis and secretion depend upon PKCα expression. Furthermore, defective platelet aggregation in response to either collagen-related peptide or thrombin could be overcome by an increase in agonist concentration. Evidence of overt bleeding, including gastrointestinal and tail bleeding, was not seen in Prkca^–/– mice. In summary, the effects of PKCα ablation on thrombus formation and granule secretion may implicate PKCα as a drug target for antithrombotic therapy.