New copolymers of vinylphosphonic acid (VPA) with 2-deoxy-2-methacrylamido-D-glucose, 4-acryloylmorpholine (4-AM), and acrylamide have been synthesized, and their antiviral activity against influenza virus has been studied in in vitro and in vivo experiments. Optimal antiviral characteristics and low cytotoxicity were exhibited by the copolymer of VPA with 4-AM, composition 56 : 44 mol %, molecular weight 33000. The polymer exhibited virus-inhibiting properties with an IC 50 = 1 μg/mL and a selectivity index of 302. Prophylactic intranasal administration of the polymer in a murine model of influenza pneumonia completely prevented the virus-induced death of animals, whereas the level of mortality in the placebo group was 90%. The results of this study indicate a high antiviral potential of polymeric compounds based on VPA.