α-Galactosylceramide (α-GalCer) is a ligand of invariant Vα14⁺ NKT cells and is presented by CD1d molecule on APC. NKT cells produce a large amount of Th1 and Th2 cytokines in response to α-GalCer-presented APC. In this study, we assessed whether α-GalCer could act as an effective nasal vaccine adjuvant for mucosal vaccine that would be capable of inducing systemic as well as mucosal immune responses. When α-GalCer was administered with OVA via the intranasal route to C57BL/6 and BALB/c mice, significant OVA-specific mucosal secretory IgA, systemic IgG, and CTL responses were induced with mixed Th1 and Th2 cytokine profiles seen in both strains of mice. Interestingly, as BALB/c mice were intranasally immunized with PR8 hemagglutinin Ag isolated from influenza virus A/PR/8/34 together with α-GalCer, significant protection was afforded against influenza viral infection. When α-GalCer was coimmunized with a replication-deficient live adenovirus to BALB/c mice, it significantly induced both humoral and cellular immune responses. In addition, intranasal administration of OVA with α-GalCer showed complete protection against EG7 tumor challenge in C57BL/6. The adjuvant effects induced by intranasal coadministration with α-GalCer were blocked in CD1d^-/- mice, indicating that the immune responses were exclusively mediated by CD1d molecule on APC. Most interestingly, intranasally coadministered α-GalCer activated naive T cells and triggered them to differentiate into functional effector T cells when CFSE-labeled OT-1 cells were adoptively transferred into syngeneic mice. Overall, our results are the first to show that α-GalCer can act as a nasal vaccine adjuvant inducing protective immune responses against viral infections and tumors.