Platelet aggregation is essential for hemostasis but can also cause myocardial infarction and stroke. A key but poorly understood step in platelet activation is the shift of the principal adhesive receptor, α_IIbβ₃ integrin, from a low- to high-affinity state for its ligands, a process that enables adhesion and aggregation. In response to stimulation of heterotrimeric guanosine triphosphate–binding protein or immunoreceptor tyrosine-based activation motif–coupled receptors, phospholipases cleave membrane phospholipids to generate lipid and soluble second messengers. An essential role in platelet activation has been established for phospholipase C (PLC) but not for PLD and its product phosphatidic acid. Here, we report that platelets from Pld1^−/− mice displayed impaired α_IIbβ₃ integrin activation in response to major agonists and defective glycoprotein Ib–dependent aggregate formation under high shear conditions. These defects resulted in protection from thrombosis and ischemic brain infarction without affecting tail bleeding times. These results indicate that PLD1 may be a critical regulator of platelet activity in the setting of ischemic cardiovascular and cerebrovascular events.