Studies of interleukin (IL)-33 reveal a number of pleiotropic properties. Here, we report that IL-33 has immunoadjuvant effects in a human papilloma virus (HPV)–associated model for cancer immunotherapy where cell-mediated immunity is critical for protection. Two biologically active isoforms of IL-33 exist that are full-length or mature, but the ability of either isoform to function as a vaccine adjuvant that influences CD4 T helper 1 or CD8 T-cell immune responses is not defined. We showed that both IL-33 isoforms are capable of enhancing potent antigen-specific effector and memory T-cell immunity in vivo in a DNA vaccine setting. In addition, although both IL-33 isoforms drove robust IFN-γ responses, neither elevated secretion of IL-4 or immunoglobulin E levels. Further, both isoforms augmented vaccine-induced antigen-specific polyfunctional CD4⁺ and CD8⁺ T-cell responses, with a large proportion of CD8⁺ T cells undergoing plurifunctional cytolytic degranulation. Therapeutic studies indicated that vaccination with either IL-33 isoform in conjunction with an HPV DNA vaccine caused rapid and complete regressions in vivo. Moreover, IL-33 could expand the magnitude of antigen-specific CD8⁺ T-cell responses and elicit effector–memory CD8⁺ T cells. Taken together, our results support the development of these IL-33 isoforms as immunoadjuvants in vaccinations against pathogens, including in the context of antitumor immunotherapy. Cancer Res; 74(6); 1789–800. ©2014 AACR.