FcγRIIA-mediated platelet activation is essential in heparin-induced thrombocytopenia (HIT) and other immune-mediated thrombocytopenia and thrombosis disorders. There is considerable inter-individual variation in platelet FcγRIIA activation, the reasons for which remain unclear. We hypothesized that genetic variations between FcγRIIA hyper- and hypo-responders regulate FcγRIIA-mediated platelet reactivity and influence HIT susceptibility. Using unbiased genome-wide expression profiling, we observed that human hypo-responders to FcγRIIA activation showed higher platelet T-cell ubiquitin ligand-2 (TULA-2) mRNA expression than hyper-responders. siRNA-mediated knockdown of TULA-2 resulted in hyper-phosphorylation of spleen tyrosine kinase (Syk) following FcγRIIA activation in HEL cells. Significantly, we found miR-148a-3p targeted and inhibited both human and mouse TULA-2 mRNA. Inhibition of miR-148a in FcγRIIA transgenic mice up-regulated the TULA-2 level and reduced FcγRIIA- and GPVI-mediated platelet αIIbβ3 activation and calcium mobilization. Anti-miR-148a also reduced thrombus formation following intravascular platelet activation via FcγRIIA. These results show that TULA-2 is a target of miR-148a-3p and TULA-2 serves as a negative regulator of FcγRIIA-mediated platelet activation. This is also the first study to show the effects of in vivo miRNA inhibition on platelet reactivity. Our work suggests that modulating miR-148a expression is a potential therapeutic approach for thrombosis.