The role of anti-tumor immune responses in oncolytic adenoviral therapy has not been well studied due to lack of efficacious tumor model in immunocompetent mice. Here, we evaluated the contributions of immune components to the therapeutic effects of oncolytic adenoviruse in an immunocompetent murine tumor model permissive for infection and replication of adenovirus. We found that CD8+ T cells were critical mediator for antitumor efficacy by oncolytic adenovirus. Intratumoral viral therapy induced intensive infiltration of CD8+ T cells in tumor, increased tumor-specific IFN-γ (interferon-γ) production and CTL (cytotoxic T lymphocyte) activity of lymphocytes, and generated a long-term tumor-specific immune memory. Boosting CD8+ T cell responses by agonistic anti-4-1BB (cluster differentiation 137, CD137) antibody showed synergistic anticancer effects with oncolytic virotherapy. Our results provide insight into antitumor mechanisms of oncolytic adenovirus in addition to their direct oncolytic effect.