Platelet activation and aggregation at sites of vascular injury are essential for primary hemostasis, but are also major pathomechanisms underlying myocardial infarction and stroke. Changes in [Ca²⁺]_i are a central step in platelet activation. In nonexcitable cells, receptor-mediated depletion of intracellular Ca²⁺ stores triggers Ca²⁺ entry through store-operated calcium (SOC) channels. STIM1 has been identified as an endoplasmic reticulum (ER)–resident Ca²⁺ sensor that regulates store-operated calcium entry (SOCE) in immune cells and platelets, but the identity of the platelet SOC channel has remained elusive. Orai1 (CRACM1) is the recently discovered SOC (CRAC) channel in T cells and mast cells but its role in mammalian physiology is unknown. Here we report that Orai1 is strongly expressed in human and mouse platelets. To test its role in blood clotting, we generated Orai1-deficient mice and found that their platelets display severely defective SOCE, agonist-induced Ca²⁺ responses, and impaired activation and thrombus formation under flow in vitro. As a direct consequence, Orai1 deficiency in mice results in resistance to pulmonary thromboembolism, arterial thrombosis, and ischemic brain infarction, but only mild bleeding time prolongation. These results establish Orai1 as the long-sought platelet SOC channel and a crucial mediator of ischemic cardiovascular and cerebrovascular events.