Subcutaneous immunization with an influenza hemagglutinin (HA) vaccine can induce the production of virus-neutralizing antibodies, but not a cell-mediated immune response. Here we tested whether amphiphilic poly(gamma-glutamic acid)-graft-l-phenylalanine copolymers (gamma-PGA-NPs), which are derived from a bacterial capsular exopolymer produced by certain Bacillus natto strains, were an effective adjuvant for systemic influenza HA vaccination. Subcutaneous immunization with a mixture of HA vaccine and gamma-PGA-NPs induced higher mononuclear cell proliferation and the production of gamma-interferon (IFN-gamma), interleukin (IL)-4, and IL-6 upon HA restimulation, and enhanced not only anti-HA neutralizing antibody production but also the influenza virus-specific cell-mediated immune response, including CTL activity, compared with immunization with HA alone or a mixture of HA and aluminum adjuvant. HA vaccine with gamma-PGA-NPs protected mice against challenges with lethal doses of homologous influenza virus. The results indicate that adding gamma-PGA-NPs to the HA vaccine promotes effective protection and identifies gamma-PGA-NPs as a new, effective, and potent candidate adjuvant for a subcutaneous influenza virus vaccine.