Integrin αIIbβ3 plays a critical role in platelet function, promoting a broad range of functional responses including platelet adhesion, spreading, aggregation, clot retraction, and platelet procoagulant function. Signaling events operating downstream of this receptor (outside-in signaling) are important for these responses; however the mechanisms negatively regulating integrin αIIbβ3 signaling remain ill-defined. We demonstrate here a major role for the Src homology 2 domain-containing inositol 5-phosphatase (SHIP1) and Src family kinase, Lyn, in this process. Our studies on murine SHIP1 knockout platelets have defined a major role for this enzyme in regulating integrin αIIbβ3-dependent phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) accumulation, necessary for a cytosolic calcium response and platelet spreading. SHIP1 phosphorylation and PtdIns(3,4,5)P3 metabolism is partially regulated through Lyn kinase, resulting in an enhanced calcium flux and spreading response in Lyn-deficient mouse platelets. Analysis of platelet adhesion dynamics under physiological blood flow conditions revealed an important role for SHIP1 in regulating platelet adhesion on fibrinogen. Specifically, SHIP1-dependent PtdIns(3,4,5)P3 metabolism down-regulates the stability of integrin αIIbβ3-fibrinogen adhesive bonds, leading to a decrease in the proportion of platelets forming shear-resistant adhesion contacts. These studies define a major role for SHIP1 and Lyn as negative regulators of integrin αIIbβ3 adhesive and signaling function.