Human platelets undergo structural and functional deterioration during extracorporeal storage at either room or cold temperature, impairing their reactivity and diminishing their hemostatic effectiveness following transfusion. PECAM-1 is an inhibitory receptor on platelets that exerts its inhibitory effects via phosphorylation of tyrosine residues that lie within its cytoplasmic immunoreceptor tyrosine-based inhibitory motifs (ITIMs).The purpose of this investigation was to attempt to restore platelet reactivity by impairing the inhibitory activity of PECAM-1.To counteract PECAM-1-mediated inhibition, we developed a novel bispecific tandem single-chain variable fragment that ligates the protein tyrosine phosphatase, CD148, with PECAM-1, promoting dephosphorylation of PECAM-1 ITIMs. We then analyzed the ability of this engineered tandem single-chain variable fragment (taFv 179) to improve adhesion and aggregation responses in vitro and under conditions of flow.Addition of taFv 179 enhanced secretion, aggregation, and activation responses of both freshly isolated and stored platelets, particularly in response to weak agonists. taFv 179 also improved thrombus formation on collagen-coated surfaces under conditions of arterial flow.These findings demonstrate that enforced approximation of a phosphatase next to PECAM-1 ITIM tyrosine receptors is a novel strategy for enhancing the functionality of stored platelets, with potential implications for improving the effectiveness of platelet transfusion therapy.