Rapid change(1) and zoonotic transmission to humans(2) have enhanced the virulence of the influenza A virus (IAV)(3). Neutralizing antibodies fail to provide lasting protection from seasonal epidemics(1),(4). Furthermore, the effectiveness of anti influenza neuraminidase inhibitors has declined because of drug resistances. Drugs that can block viral attachment and cell entry independent of antigenic evolution or drug resistance might address these problems. We show that multivalent 6'-sialyllactose-polyamidoamine (6SL-PAMAM) conjugates, when designed to have well-defined ligand valencies and spacings, can effectively inhibit IAV infection. Generation 4 (G4) 6SL-PAMAM conjugates with a spacing of around 3 nm between 6SL ligands (S3-G4) showed the strongest binding to a hemagglutinin trimer (dissociation constant of 1.6 x 10(-7) M) and afforded the best inhibition of H1N1 infection. S3-G4 conjugates were resistant to hydrolysis by H1N1 neuraminidase. These conjugates protected 75% of mice from a lethal challenge with H1N1 and prevented weight loss in infected animals. The structure-based design of multivalent nanomaterials, involving modulation of nanoscale backbone structures and number and spacing between ligands, resulted in optimal inhibition of IAV infection. This approach may be broadly applicable for designing effective and enduring therapeutic protection against human or avian influenza viruses.