Thrombin is a serine protease that cleaves the peptide bonds in proteins located on the carboxyl side of arginine. Thrombin plays a central role in thromboembolic diseases, which are the major cause of mortality. The aim of the study was to estimate the effects of plant extracts on proteolytic properties of thrombin. Thrombin was incubated with polyphenol‐rich extracts from berries of Aronia melanocarpa or seeds of Vitis vinifera (0.5, 5, 50 µg/mL) and with polyphenols ((+)‐catechin, (−)‐epicatechin, gallic acid, chlorogenic acid, procyanidin B1, cyanidin, cyanidin 3‐glucoside, quercetin). The in vitro experiments showed that both extracts in all used concentrations inhibited proteolytic activity of thrombin observed as inhibition of thrombin‐induced fibrinogen polymerization, stabilized fibrin formation, and platelet aggregation. Moreover, thrombin amidolytic activity was inhibited by polyphenols belonging to the flavonoid class. Results presented in this study indicate that polyphenol‐rich extracts from berries of A. melanocarpa and seeds of V. vinifera may become promising dietary supplements in the prevention of thrombotic states. Copyright © 2012 John Wiley & Sons, Ltd.