CMDBS compounds are synthetic dextran derivatives with a random distribution of glucosyl units substituted with carboxymethyl, benzylamide, sulfonate, and sulfate groups. Fucoidans are sulfated polysaccharides extracted from brown seaweeds. CMDBS and fucoidans exhibit anticoagulant activity which depends on their chemical composition and molecular weight. Tested with purified proteins, these compounds catalyse thrombin (EC 3.4.21.5) inhibition mainly via heparin cofactor II (HCII). We investigated the mechanism involved in the anticoagulant activity of these polysaccharides relative to that of heparin. Three CMDBS with different chemical compositions were studied to evaluate the effect of sulfate and sulfonate groups on the anticoagulant activity. The fucoidan fraction was extracted from the brown seaweed Ascophylum nodosum. The clotting assays (activated partial thromboplastin time, thrombin time, prothrombin time) were significantly prolonged in the presence of CMDBS and fucoidan, which were less active than heparin. To investigate the action mechanism of these polysaccharides, thrombin generation tests (TGT) were performed on human plasma in the presence of several CMDBS and a fucoidan fraction. The results showed an inhibition of thrombin generation in contact-activated plasma in the presence of both polysaccharides, with a prolonged lag phase preceding the burst of thrombin generation. In thromboplastin-activated plasma, thrombin generation was inhibited by CMDBS and fucoidan, with a prolonged lag phase only in the presence of CMDBS. The data obtained with each polysaccharide, compared to those obtained with heparin (our study) and hirudin (published data), led to hypothesize that fucoidan could act, like heparin, by forming complexes with the inhibitor (although antithrombin (AT) in the case of heparin, and HCII for fucoidan), while CMDBS could act, like hirudin, by forming complexes with thrombin.