Abstract Vaccination is the most effective strategy to protect against infectious diseases and adjuvants are proven to be the key component in vaccines capable of improving antigen-specific immune responses. Conventional vaccine adjuvants, however, still have several limitations such as safety concerns and insufficient induction of vaccine efficacy. Here, we developed a novel adjuvant by combining poly-gamma glutamic acid (γ-PGA) with alum that is already approved for human use worldwide. The physiochemical properties, efficacy, and underlying mechanism of the PGA/Alum as an adjuvant were investigated using ovalbumin (OVA) as a model antigen in vivo and in vitro. PGA/Alum-OVA administered mice showed the robust increase of antigen-specific humoral and cellular immune responses compared to mice administered with γ-PGA-OVA or alum-OVA. Importantly, the PGA/Alum-OVA-induced antibodies had significantly higher antibody-dependent cellular cytotoxicity (ADCC) activity. The enhanced activation and antigen processing of dendritic cells (DCs) and the facilitated migration of antigen-loaded DCs from the injection sites to draining lymph nodes were observed in PGA/Alum-OVA administered mice. Finally, the efficacy of the PGA/Alum as a vaccine adjuvant was evaluated using pandemic H1N1 influenza vaccine antigen. The PGA/Alum drastically enhanced protective efficacy of the pandemic H1N1 vaccine against influenza virus infection. Taken together, PGA/Alum may be a promising adjuvant to improve the influenza vaccine efficacy.