<p>This study investigates the influence of polyphenolic impurities on the antibacterial and antioxidant activities of fucoidans extracted from <i>Fucus vesiculosus</i> collected in different seasons in the Barents Sea. Fucoidans prepared in different ways were analyzed: crude fucoidans and fucoidans isolated after removal of the polyphenolic fraction from the alga. Polyphenol-rich fractions were prepared using aqueous acetone, while fucoidans were isolated by calcium chloride extraction and ethanol precipitation. The chemical composition, antioxidant properties and antibacterial activity of all samples were evaluated. The results showed significant seasonal and process-based variation in polyphenol content, which directly affected the bioactivity of fucoidan. The study revealed a strong correlation between the radical scavenging activity of fucoidans and their polyphenol content. Crude fucoidan samples showed strong dose-dependent antibacterial activity. In contrast, fucoidan samples with minimal polyphenol content showed significantly reduced activity. <i>Eschericia coli</i> and <i>Staphylococcus aureus</i> were sensitive to all fucoidan samples, with minimum inhibitory concentrations ranging from 2 to 6 mg mL^-1. Among the cultures analyzed, <i>E. coli</i> showed no susceptibility to the antibacterial effects of the polyphenol-rich fractions, in stark contrast to <i>S. aureus</i> and <i>B. cereus</i>, which showed remarkable sensitivity. This study highlights the critical role of polyphenolic impurities in the bioactivity of fucoidans. These findings provide a basis for optimizing extraction protocols to yield fucoidans with tailored antibacterial and antioxidant properties relevant to pharmaceutical and nutraceutical applications.</p>