Focal adhesion kinase (FAK) plays a key role in mediating signaling downstream of integrins and growth factor receptors. In this study, we determined the roles of FAK in vivo by generating a megakaryocyte lineage-specific FAK-null mouse (Pf4-Cre/FAK-floxed). Megakaryocyte and platelet FAK expression was ablated in Pf4-Cre/FAK-floxed mice without affecting expression of the FAK homologue PYK2, although PYK2 phosphorylation was increased in FAK-/- megakaryocytes in response to fibrinogen. Megakaryopoiesis is greatly enhanced in Pf4-Cre/FAK-floxed mice, with significant increases in megakaryocytic progenitors (CFU-MK), mature megakaryocytes, megakaryocyte ploidy, and moderate increases in resting platelet number and platelet recovery following a thrombocytopenic stress. Thrombopoietin (Tpo)-mediated activation of Lyn kinase, a negative regulator of megakaryopoiesis, is severely attenuated in FAK-null megakaryocytes compared with wild-type controls. In contrast, Tpo-mediated activation of positive megakaryopoiesis regulators such as ERK1/2 and AKT is increased in FAK-null megakaryocytes, providing a plausible explanation for the observed increases in megakaryopoiesis in these mice. In Pf4-Cre/FAK-floxed mice, rebleeding times are significantly increased, and FAK-null platelets exhibit diminished spreading on immobilized fibrinogen. These studies establish clear roles for FAK in megakaryocyte growth and platelet function, setting the stage for manipulation of this component of the Tpo signaling apparatus for therapeutic benefit.