FXIIa-IN-3 (Compound 8) is a potent and selective Factor XIIa ( FXIIa ) inhibitor, with an IC 50 of 0.045 μM. FXIIa-IN-3 also exhibits a substantial margin of selectivity against related serine proteases, including FXIa, FXa, and FIXa. FXIIa-IN-3 can be used for the research of thromboembolic diseases.