The role of c-Jun NH₂-terminal kinase 1 (JNK1) in hemostasis and thrombosis remains unclear. We show here, with JNK1-deficient (JNK1^−/−) mice, that JNK1 plays an important role in platelet biology and thrombus formation. In tail-bleeding assays, JNK1^−/− mice exhibited longer bleeding times than wild-type mice (396 ± 39 seconds vs 245 ± 32 seconds). We also carried out in vitro whole-blood perfusion assays on a collagen matrix under arterial shear conditions. Thrombus formation was significantly reduced for JNK1^−/− platelets (51%). In an in vivo model of thrombosis induced by photochemical injury to cecum vessels, occlusion times were 4.3 times longer in JNK1^−/− arterioles than in wild-type arterioles. Moreover, in vitro studies carried out in platelet aggregation conditions demonstrated that, at low doses of agonists, platelet secretion was impaired in JNK1^−/− platelets, leading to altered integrin αIIbβ3 activation and reduced platelet aggregation, via a mechanism involving protein kinase C. JNK1 thus appears to be essential for platelet secretion in vitro, consistent with its role in thrombus growth in vivo. Finally, we showed that ERK2 and another isoform of JNK affect platelet aggregation through 2 pathways, one dependent and another independent of JNK1.