Platelet responses to collagen are mediated by the combined actions of the integrin α₂β₁, which serves as a major collagen-binding receptor, and the GPVI/FcRγ-chain complex, which transmits collagen-specific activation signals into the cell interior through the action of an immunoreceptor tyrosine-based activation motif within the cytoplasmic domain of the FcRγ-chain. Despite much progress in identifying components of the signaling pathway responsible for collagen-induced platelet activation, virtually nothing is known about the regulatory elements that modulate this important hemostatic event. PECAM-1, a recently recognized member of the inhibitory receptor family, contains a functional immunoreceptor tyrosine-based inhibitory motif within its cytoplasmic domain that, when tyrosine phosphorylated, recruits and activates the protein–tyrosine phosphatase, SHP-2. To test the hypothesis that PECAM-1 functions to regulate GPVI/FcRγ-chain–mediated platelet activation, the responses of wild-type versus PECAM-1–deficient murine platelets to GPVI-specific agonists were compared. Four distinct GPVI/FcRγ-chain–dependent responses were found to be significantly exaggerated in platelets derived from PECAM-1–deficient mice, including Mg⁺⁺-independent adhesion to immobilized fibrillar collagen, collagen-induced platelet aggregation, platelet aggregation induced by the GPVI-specific agonist collagen-related peptide, and GPVI/FcRγ-chain–induced dense granule secretion. Together, these data provide compelling evidence that PECAM-1 modulates platelet responses to collagen, and they implicate this novel member of the inhibitory receptor family in the regulation of primary hemostasis.