Understanding noncanonical mechanisms of platelet activation represents an important challenge for the identification of novel therapeutic targets in bleeding disorders, thrombosis, and cancer. We previously reported that galectin-1 (Gal-1), a β-galactoside-binding protein, triggers platelet activation in vitro. Here we investigated the molecular mechanisms underlying this function and the physiological relevance of endogenous Gal-1 in hemostasis. Mass spectrometry analysis, as well as studies using blocking antibodies against the anti-α(IIb) subunit ofα(IIb)β(3) integrin or platelets from patients with Glanzmann's thrombasthenia syndrome (α(IIb)β(3) deficiency), identified this integrin as a functional Gal-1 receptor in platelets. Binding of Gal-1 to platelets triggered the phosphorylation of β(3)-integrin, Syk, MAPKs, PI3K, PLCγ2, thromboxane (TXA(2)) release, and Ca(2+) mobilization. Not only soluble but also immobilized Gal-1 promoted platelet activation. Gal-1-deficient (Lgals1(-/-)) mice showed increased bleeding time (P<0.0002, knockout vs. wild type), which was not associated with an abnormal platelet count. Lgals1(-/-) platelets exhibited normal aggregation to PAR4, ADP, arachidonic acid, or collagen but abnormal ATP release at low collagen concentrations. Impaired spreading on fibrinogen and clot retraction with normal levels of α(IIb)β(3) was also observed in Lgals1(-/-) platelets, indicating a failure in the "outside-in" signaling through this integrin. This study identifies a noncanonical mechanism, based on galectin-integrin interactions, for regulating platelet activation.