Megakaryocytes (MKs) are bone marrow–derived cells that are primarily responsible for generating platelets for the maintenance of hemostasis. Although MK can variably express major histocompatibility complex (MHC) class I and II molecules during their differentiation, little is known whether they can elicit nonhemostatic immune functions such as T-cell activation. Here, we demonstrate that mature CD34⁻ MHC class II⁻ CD41⁺ MKs can endocytose exogenous ovalbumin (OVA) and proteolytically generate its immunogenic peptide ligand, which is crosspresented on their surface in association with MHC class I molecules. This crosspresentation triggered in vitro and in vivo OVA-specific CD8⁺ T-cell activation and proliferation. In addition, the OVA-MHC class I complexes were transferred from MK to pro-platelets upon thrombopoiesis in vitro. MK could also present endogenous MK-associated (CD61) peptides to activate CD61-specific CD8⁺ T cells and mediate immune thrombocytopenia in vivo. These results suggest that, in addition to their hemostatic role, mature MKs can significantly affect antigen-specific CD8⁺ T-cell responses via antigen presentation and are able to spread this immunogenic information through platelets.