Background and Purpose: Mechanical properties of thromboemboli play an important role in the efficacy of endovascular thrombectomy (EVT) for acute ischemic stroke. However, very limited data on mechanical properties of human stroke thrombi are available. We aimed to mechanically characterize thrombi retrieved with EVT, and to assess the relationship between thrombus composition and thrombus stiffness. Methods: Forty-one thrombi from 19 patients with acute stroke who underwent EVT between July and October 2019 were mechanically analyzed, directly after EVT. We performed unconfined compression experiments and determined tangent modulus at 75% strain (E^t75) as a measure for thrombus stiffness. Thrombi were histologically analyzed for fibrin/platelets, erythrocytes, leukocytes, and platelets, and we assessed the relationship between histological components and E^t75with univariable and multivariable linear mixed regression. Results: Median E^t75was 560 (interquartile range, 393-1161) kPa. In the multivariable analysis, fibrin/platelets were associated with increased E^t75(aβ, 9 [95% CI, 5 to 13]) kPa, erythrocytes were associated with decreased E^t75%(aβ,-9 [95% CI,-5 to-13]) kPa. We found no association between leukocytes and E^t75. High platelet values were strongly associated with increased E^t75(aβ, 56 [95% CI, 38-73]). Conclusions: Fibrin/platelet content of thrombi retrieved with EVT for acute ischemic stroke is strongly associated with increased thrombus stiffness. For thrombi with high platelet values, there was a very strong relationship with thrombus stiffness. Our data provide a basis for future research on the development of next-generation EVT devices tailored to thrombus composition.