In 2009, the global outbreak of an influenza pandemic emphasized the need for an effective vaccine adjuvant. In this study, we examined the efficacy of poly-γ-glutamic acid/chitosan (PC) nanogel as an adjuvant for the influenza vaccine. PC nanogel significantly enhanced antigen-specific cross-presentation and cytotoxic T lymphocyte (CTL) activity. Compared with alum, the protective efficacy of the pandemic H1N1 influenza (pH1N1) vaccine was substantially increased by PC nanogel, with increased hemagglutination-inhibition titers, CTL activity, and earlier virus clearance after homologous and heterosubtypic [A/Philippines/2/82 (H3N2)] virus challenges. However, CD8+ T cell-depleted mice displayed no protection against the heterosubtypic virus challenge after immunization with PC nanogel-adjuvanted pH1N1 vaccine. We also observed that using PC nanogel as a vaccine adjuvant had a dose-sparing effect and significantly enhanced the long-lasting protection of the pH1N1 vaccine. Together, these results suggest that PC nanogel is a promising vaccine adjuvant that could broadly prevent influenza virus infection.