Inappropriate platelet activation remains a major cause of cardiovascular and cerebrovascular diseases. Most agonists activate platelets through G protein-coupled receptors (GPCRs). However, questions remain about mechanisms that provide negative feedback towards activated GPCRs to limit platelet activation and thrombus formation. Here we provide the first evidence that GPCR kinase 6 (GRK6) serves this role in platelets, using GRK6-/- mice generated by CRISPR-Cas9 genome editing to examine the consequences of GRK6 knockout on GPCR-dependent signaling. Hemostatic thrombi formed in GRK6-/- mice are larger than in WT controls during the early stages of thrombus formation, with a rapid increase of platelet accumulation at site of injury. Platelet activation in the absence of GRK6 is enhanced, but in an agonist-selective manner. Responses to PAR4 agonist peptide or ADP stimulation in GRK6-/- platelets are increased compared to WT control littermates, while the response to TxA2 is normal. Underlying these changes in GRK6-/- platelets is an increase in Ca2+ mobilization, Akt activation, and granule secretion. Furthermore, deletion of GRK6 in human MEG-01 cells causes an increase in Ca2+ response and PAR1 surface expression in response to thrombin. Finally, we show that in human platelets, platelet activation in response to thrombin causes an increase in binding of GRK6 to PAR1, as well as an increase of the phosphorylation of PAR1. Deletion of GRK6 in MEG-01 cells causes a decrease in PAR1 phosphorylation. Collectively, these observations, for the first time, show that 1) GRK6 regulates the hemostatic response to injury by thrombin and ADP, 2) it mediates platelet activation by reducing PAR1/4- and P2Y12-dependent signaling, and 3) GRK6 limits the rate of platelet activation during early stage of thrombus growth and helps prevent inappropriate platelet activation. Disclosures No relevant conflicts of interest to declare.