Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a rare adverse effect of COVID-19 adenoviral vector vaccines1–3. VITT resembles heparin-induced thrombocytopenia (HIT) as it is associated with platelet-activating antibodies against platelet factor 4 (PF4)4; however, patients with VITT develop thrombocytopenia and thrombosis without heparin exposure. The objective of this study was to determine the binding site on PF4 of antibodies from patients with VITT. Using alanine scanning mutagenesis5, we determined the binding of VITT anti-PF4 antibodies (n=5) was restricted to 8 surface amino acids, all of which were located within the heparin binding site on PF4, and the binding was inhibited by heparin. In contrast, HIT sampled (n=10) bound to amino acids corresponding to 2 different sites on PF4. Using biolayer interferometry, we demonstrated VITT anti-PF4 antibodies had a stronger binding response against PF4 and PF4/heparin complexes than HIT antibodies; albeit, with similar dissociation rates. Our data indicates VITT antibodies can mimic the effect of heparin by binding to a similar site on PF4, allowing PF4 tetramers to cluster and form immune complexes, which in turn cause FcγRIIa-dependent platelet activation. These results provide an explanation for VITT antibody-induced platelet activation that could contribute to thrombosis.