Introduction: In the present study, we aimed to investigate whether the measurements of the fibrinogen-dependent clot firmness in FIBTEM-one of the rotational thromboelastometry (ROTEM) assays-during cardiopulmonary bypass (CPB) accurately predict the values after protamine administration. Materials and Methods: In this single-center retrospective observational study, we analyzed a database of patients undergoing on-pump surgeries between May 2022 and February 2024. We included patients in whom an intraoperative ROTEM examination during CPB and a follow-up examination directly after protamine infusion were performed. We excluded patients who received either fibrinogen concentrate or fresh frozen plasma between the two examinations. FIBTEM A5 values in both examinations were compared. The Wilcoxon signed rank test was used to compare non-normally distributed data. Linear regression analysis was used to investigate the relationship between the on-pump and postprotamine FIBTEM A5 values. Results: Seventy patients were included in the statistical analysis. There was a slight but statistically significant difference between FIBTEM A5 during CPB (median 16.0 mm, IQR 10.0-22.0) and after protamine administration (median 15.5 mm, IQR 8.7-22.3, p = 0.021). However, in linear regression analysis, FIBTEM A5 values during the last 30 min of the extracorporeal circulation were a significant predictor of FIBTEM A5 after protamine administration ( R  = 0.902, R 2  = 0.813, adjusted R 2  = 0.810, F(df regression, df residual) = 295.980, p < 0.001). The equation: y = 0.911 x   +  1 describes the line of best fit. Conclusions: Within the normal range limits, there is a very strong correlation between on-pump FIBTEM A5 values and FIBTEM A5 values after protamine administration.