The structure of fucoidan isolated from Laminaria hyperborea was elucidatedand chemically tailored in order to obtain a clear structure−function relationship on bioactiveproperties with a minimal amount of variations among the tested molecules. Analysis revealeda sugar composition of 97.8% fucose and 2.2% galactose. Analysis of the glycosidic linkagesshowed (1→3)-α-L-fuco-pyranose (31.9%) to be the dominant residue, followed by 1→2-linked (13.2%) and 1→4-linked (7.7%) fuco-pyranose as well as a high degree of branching(22.4%). Inductively coupled plasma mass spectrometry (ICP-MS) revealed a sulfate contentof 53.8% (degree of sulfation (DS) = 1.7). Raman spectroscopy determined SO4 located axialat 4C and equatorial at 2C as well as an absence of acetylation. SEC-MALS analysisdetermined a high molecular weight (Mw = 469 kDa), suggesting a highly flexible main chainwith short side chains. Both chemical shifts of the fucoidan, proton, and carbon were assignedby NMR and revealed a highly heterogeneous structure in terms of glycosidic linkages.Bioactivity was assessed using a lepirudin-based whole blood model. The immediate responsesby coagulation and complement cascades were measured by prothrombine factor 1 and 2(PTF1.2) and the terminal complement complex (TCC). Cytokines involved in inflammation were detected in a 27-plexcytokine assay. Fucoidan with a high Mw and DS inhibited coagulation, complement, and the cytokines PDGF-BB, RANTES,and IP-10, while activating MCP-1. These effects were obtained at the concentration of 1000 ug/mL and partly at 100 ug/mL.In low concentrations (10 ug/mL), a coagulation stimulating effect of highly sulfated fucoidans (DS = 1.7, Mw = 469 kDa or20.3) was obtained. These data point to a multitude of effects linked to the sulfation degree that needs further mechanisticexploration.