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Abstract Thrombus fragmentation during endovascular stroke treatment, such as mechanical thrombectomy, leads to downstream emboli, resulting in poor clinical outcomes. Clinical studies suggest that fragmentation risk is dependent on clot composition. This current study presents the first experimental characterization of the fracture properties of blood clots, in addition to the development of a predictive model for blood clot fragmentation. A bespoke experimental test-rig and compact tension specimen fabrication has been developed to measure fracture toughness of thrombus material. Fracture tests are performed on three physiologically relevant clot compositions: a high fibrin 5% H clot, a medium fibrin 20% H clot, a low-fibrin 40% H clot. Fracture toughness is observed to significantly increase with increasing fibrin content, i.e. red blood cell-rich clots are more prone to tear during loading compared to the fibrin-rich clots. Results also reveal that the mechanical behaviour of clot analogues is significantly different in compression and tension. Finite element cohesive zone modelling of clot fracture experiments show that fibrin fibres become highly aligned in the direction perpendicular to crack propagation, providing a significant toughening mechanism. The results presented in this study provide the first characterization of the fracture behaviour of blood clots and are of key importance for development of next-generation thrombectomy devices and clinical strategies.