Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a rare complication of adenovirus vector-based COVID-19 vaccines. VITT is associated with markedly raised levels of D-dimer yet how VITT modulates the fibrinolytic system is unknown. We aimed to compare changes in fibrinolytic activity in plasma from patients with VITT, patients diagnosed with venous thromboembolism post-vaccination but without VITT (VTE-no VITT), and healthy vaccinated controls.Plasma levels of plasmin-antiplasmin (PAP) complexes, plasminogen, and alpha2-anti-plasmin (A2AP) from 10 patients with VITT, 10 patients with VTE-no VITT, and 14 healthy vaccinated controls were evaluated by ELISA and/or Western blotting. Fibrinolytic capacity was evaluated by quantitating PAP levels at baseline and after ex vivo plasma stimulation with 50nM tissue-type plasminogen activator (tPA) or urokinase (uPA) for 5 minutes.Baseline PAP complex levels in control and VTE-no VITT individuals were similar but were ∼7-fold higher in plasma from VITT patients (p<0.0001). VITT samples also revealed consumption of A2AP and fibrinogenolysis consistent with a hyperfibrinolytic state. Of interest, VITT plasma produced significantly higher PAP levels after ex vivo treatment with tPA, but not uPA compared to the other groups, indicative of increased fibrinolytic potential. This was not due to D-dimer as addition of D-dimer to VTE-no VITT plasma failed to potentiate tPA-induced PAP levels.A marked hyperfibrinolytic state occurs in patients with VITT, evidenced by marked elevations in PAP, A2AP consumption and fibrinogenolysis. An unidentified plasma cofactor that selectively potentiates tPA-mediated plasminogen activation also appears to exist in the plasma of patients with VITT.