© 2018 The Royal Society of Chemistry. Among a series of chemically synthesized fucoidan derivatives (1-9), 5 was found for the first time to bind to influenza virus hemagglutinins (HAs) and inhibit hemagglutination activity. In addition, a designed C3-symmetric tripodal ligand 10, synthesized with three sulfated oligofucoside moieties of 5, exhibited much greater hemagglutination inhibition activity than 5. A plaque assay using MDCK cells demonstrated that 10 effectively inhibited influenza virus infection.