LPS activates platelets through TLR4, aiding productive sepsis, with stimulated splicing and translation of stored heteronuclear pro-IL-1β RNA. Although the IL-1R type 1 (IL-1R1) receptor for IL-1 shares downstream components with the TLR4 receptor, platelets are not known to express IL-1R1, nor are they known to respond to this cytokine. We show by flow cytometry and Western blotting that platelets express IL-1R1, and that IL-1β and IL-1α stimulate heteronuclear I-1β splicing and translation of the newly made mRNA in platelets. Platelets also respond to the IL-1β they make, which is exclusively associated with shed microparticles. Specific blockade of IL-1R1 with IL-1R antagonist suppressed platelet stimulation by IL-1, so IL-1β stimulates its own synthesis in an autocrine signaling loop. Strikingly, IL-1R antagonist inhibition, pharmacologic or genetic suppression of pro-IL-1β processing to active cytokine by caspase-1, or blockade of de novo protein synthesis also blocked LPS-induced IL-1β mRNA production. Robust stimulation of platelets by LPS therefore also required IL-1β amplification. Activated platelets made IL-1β in vivo as IL-1β rapidly accumulated in occluded murine carotid arteries by posttranscriptional RNA splicing unique to platelets. We conclude that IL-1β is a platelet agonist, that IL-1β acts through an autocrine stimulatory loop, that an IL-1β autocrine loop is required to amplify platelet activation by LPS, and that platelets immobilized in occlusive thrombi are activated over time to produce IL-1β. IL-1 is a new platelet agonist that promotes its own synthesis, connecting thrombosis with immunity.