Background: Reactive oxygen species (ROS) contribute to platelet hyperactivation during aging. Several oxidative pathways and antioxidant enzymes have been implicated; however, their mechanistic contributions during aging remain elusive. We hypothesized that mitochondria are an important source of platelet ROS and that mitochondrial SOD2 (superoxide dismutase) protects against mitochondrial ROS-driven platelet activation and thrombosis during aging. Methods: We studied littermates of platelet-specific SOD2-knockout (SOD2 ^fl/fl Pf4Cre, pSOD2-KO) and control (SOD2 ^fl/fl ) mice at young (4–5 months) or old (18–20 months) ages. We examined agonist-induced platelet activation, platelet-dependent thrombin generation potential, and susceptibility to in vivo thrombosis. Results: Platelet α _{II b} β3 activation, aggregation, and adhesion were increased to similar extents in aged mice of both genotypes compared with young mice. In contrast, the age-dependent increases in mitochondrial and total cellular ROS, calcium elevation, and phosphatidylserine exposure were augmented in platelets from pSOD2-KO mice compared with control mice. Aged pSOD2-KO mice showed increased platelet-dependent thrombin generation compared with aged control mice. In vivo, aged pSOD2-KO mice exhibited enhanced susceptibility to carotid artery and pulmonary thrombosis compared to aged control mice. Adoptive transfer of platelets from aged pSOD2-KO but not aged control mice increased thrombotic susceptibility in aged host mice, suggesting a prothrombotic effect of platelet pSOD2 deficiency. Treatment with avasopasem manganese (GC4419), a SOD mimetic, decreased platelet mitochondrial pro-oxidants, cellular ROS levels, and inhibited procoagulant platelet formation and arterial thrombosis in aged mice. Conclusions: Platelet mitochondrial ROS contributes to age-related thrombosis and endogenous SOD2 protects from platelet-dependent thrombin generation and thrombosis during aging.