The aim of this study was to elucidate some mechanisms of radical scavenging and the anti-inflammatory, anti-hyperglycemic, and anti-coagulant bioactivities of high molecular weight fucoidan from Fucus vesiculosus in several in vitro models . Fucoidan has displayed potent 1 , 1-diphenyl-2-picryl hydrazil radical scavenging and reduction power activities . It significantly inhibits the cyclooxygenase-2 (COX-2) enzyme (IC50 4.3 µg mL-1) with a greater selectivity index (lg (IC80 COX-2/IC80COX-1), -1.55) than the synthetic non-steroidal anti-inflammatory drug indomethacin (lg (IC80 COX-2/IC80COX-1), -0.09). A concentration-dependent inhibition of hyaluronidase enzyme with an IC50 of 2.9 µg mL-1 was observed . Fucoidan attenuated the lipopolysaccharide-induced expression of mitogen-activated protein kinase p38 . Our findings suggest that the inhibition of dipeptidyl peptidase-IV (DPP-IV) (IC50 1.11 µg mL-1) is one of the possible mechanisms involved in the anti-hyperglycemic activity of fucoidan . At a concentration of 3.2 µg mL-1, fucoidan prolongs the activated partial thromboplastin time and thrombin time by 1.5-fold and 2.5-fold compared with a control, respectively . A significant increase of prothrombin time was observed after the concentration of fucoidan was increased above 80 µg mL-1 . This evidenced that fucoidan may have an effect on intrinsic/common pathways and little effect on the extrinsic mechanism . This study sheds light on the multiple pathways of the bioactivities of fucoidan . As far as we know, the inhibition of hyaluronidase and DPP-IV by high molecular fucoidan was studied for the first time in this work . Our results and literature data suggest that molecular weight, sulfate content, fucose content, and polyphenols may contribute to these activities . It seems that high molecular weight fucoidan has promising therapeutic applications in different pharmacological settings . Anti-oxidant, anti-inflammatory and anti-coagulant drugs have been used for the management of complications of COVID19 . Taken as a whole, fucoidan could be considered as a prospective candidate for the treatment of patients with COVID19; however, additional research in this field is required.