Rationale : Platelets contain abundant thymidine phosphorylase (TYMP), which is highly expressed in diseases with high risk of thrombosis, such as atherosclerosis and type II diabetes mellitus. Objective : To test the hypothesis that TYMP participates in platelet signaling and promotes thrombosis. Methods and Results : By using a ferric chloride (FeCl 3 )–induced carotid artery injury thrombosis model, we found time to blood flow cessation was significantly prolonged in Tymp −/− and Tymp +/− mice compared with wild-type mice. Bone marrow transplantation and platelet transfusion studies demonstrated that platelet TYMP was responsible for the antithrombotic phenomenon in the TYMP-deficient mice. Collagen-, collagen-related peptide–, adenosine diphosphate-, or thrombin-induced platelet aggregation were significantly attenuated in Tymp +/− and Tymp −/− platelets, and in wild type or human platelets pretreated with TYMP inhibitor KIN59. Tymp deficiency also significantly decreased agonist-induced P-selectin expression. TYMP contains an N-terminal SH3 domain-binding proline-rich motif and forms a complex with the tyrosine kinases Lyn, Fyn, and Yes in platelets. TYMP-associated Lyn was inactive in resting platelets, and TYMP trapped and diminished active Lyn after collagen stimulation. Tymp/Lyn double haploinsufficiency diminished the antithrombotic phenotype of Tymp +/− mice. TYMP deletion or inhibition of TYMP with KIN59 dramatically increased platelet-endothelial cell adhesion molecule 1 tyrosine phosphorylation and diminished collagen-related peptide– or collagen-induced AKT phosphorylation. In vivo administration of KIN59 significantly inhibited FeCl 3 -induced carotid artery thrombosis without affecting hemostasis. Conclusions : TYMP participates in multiple platelet signaling pathways and regulates platelet activation and thrombosis. Targeting TYMP might be a novel antiplatelet and antithrombosis therapy.