Objective: Little is known about the coagulation activity of factor XIII (FXIII) during resuscitation for hemorrhagic shock and the effects of plasma transfusions. We performed a single-center observational study to evaluate the changes in FXIII activity during resuscitation for hemorrhagic shock.Patient and Methods: Twenty-three adult patients with hemorrhagic shock were enrolled in this study. Blood samples were drawn upon arrival (T1), at the time of hemostasis completion (T2), and on day 2 (T3). Baseline and changes in FXIII activity and the proportion of patients with adequate levels of FXIII activity (FXIII activity >70%) were evaluated. The effects of plasma transfusion on these parameters were also investigated.Results: At T1, the median (interquartile range) FXIII activity was 53% (47–85%), which did not increase (T1 vs. T3: 53% [47–85%] vs. 63% [52–70%], P=0.8766). The proportion of patients with adequate FXIII activity decreased throughout the resuscitation period (T1, T2, and T3: 30, 34, and 21%, respectively). Plasma transfusion did not affect FXIII activity (T1 vs. T2, 66.4% [23.4] vs. 70.0% [16.2%], P=0.3956; T2 vs. T3, 72.0% [19.5] vs. 63.5% [8.6%], P=0.1161) or the proportion of adequate levels of FXIII activity at 44% at T2 and 27% at T3.Conclusion: FXIII activity is low during the early phase of a hemorrhagic shock. Even with plasma transfusion, FXIII levels were not adequately maintained throughout resuscitation.