Thrombocytopenia-absent radius (TAR) syndrome is caused by RBM8A insufficiency. We generated megakaryocyte-specific knockout (KO) mice that exhibited marked thrombocytopenia, internal hemorrhage, and splenomegaly, providing evidence that genetic deficiency of causes a disorder of platelet production. KO mice accumulated low-ploidy immature megakaryocytes in the bone marrow and exhibited defective platelet activation and aggregation. Accordingly, depletion of Y14 (RBM8A) in human erythroleukemia (HEL) cells compromised phorbol-ester-induced polyploidization. Notably, Y14/RBM8A deficiency induced both p53 and p21 in megakaryocytes and HEL cells. Treatment with a p53 inhibitor restored differentiation of KO megakaryocytes and unexpectedly activated Y14 expression in HEL cells knockout partially restored megakaryocyte differentiation by reversing cell-cycle arrest and increased platelet counts of KO, indicating that excess p53 in part accounts for thrombocytopenia in TAR syndrome. This study provides evidence for the role of the Y14-p53 circuit in platelet production and a potential therapeutic strategy.