Anticoagulant potential of Halymenia red seaweeds sulfated polysaccharides (SPs), which are distributed along the Ceará coast, Brazil, to act as inhibitors of thrombin generation (TG) is unknown. This study aimed to examine the in vitro inhibition of TG by Halymenia sp. SPs. Papain incubation yielded extract (41.6%) containing three polysaccharidic fractions (F I, F II and F III, eluted at 0.5, 0.75 and 1 M NaCl) separated by DEAE-cellulose anion-exchange chromatography, with differences among the relative proportions of sulfate (20.34-35.12%), total sugars (30.02-62.77%) and no traces of proteins. Procedures in 5% agarose / 6% polyacrylamide gel electrophoresis revealed complex glycans migrating as negatively charged components and high molecular distribution (> 100 kDa), respectively. These techniques, when associated with the use of stains-all, nonSPs were also suggested. Blood coagulation was modified by extract (19.6 IU mg-1) and fractions (3.96, 7.88 and 2.04 IU mg-1, respectively)  dependent on the sulfation degree of the SPs using the activated partial thromboplastin time test vs. heparin (193 IU mg-1), while Haymenia sp. SPs-treated human plasma did not altered the prothrombin time. In TG continuous assay in 60-fold diluted human plasma using chromogenic substrate showed that extract and F II inhibited concentration-dependent and sulfation both intrinsic and extrinsic coagulation pathways, with 50% efficacy on cephalin-induced TG already at concentrations of 4.1 and 8.3 µg of extract vs. heparin that had ability to abolish TG at 20-fold lower concentration. The results reveal to a potential of the Halymenia sp. SPs obtained at a different period of the year to prevent thrombosis in vitro and opportunities to use this species in industry could be better recognized in studies future.