Cancer treatment combining chemotherapy and immunotherapy has been vigorously exploited to further improve cancer therapeutic efficacy. This study investigated a new chemoimmunotherapy approach utilizing hydrogel as a local anti-cancer drug delivery system. Chitosan hydrogel containing doxorubicin (CH-DOX) and vaccinia virus vaccine expressing Sig/E7/LAMP-1 (Vac-Sig/E7/LAMP-1) were used as chemoimmunotherapeutic agents. It was found that intratumoral injection of CH-DOX effectively inhibited tumor growth itself and, in addition, exhibited a synergistic antitumor effect in combination with a vaccinia virus-based vaccine. This combination did not decrease but rather increased the number of tumor-specific CD8⁺ T cells primed by vaccinia virus-mediated vaccination; the resulting antitumor effects were further improved up to 60 days as compared with monotherapy after tumor challenge, and the survival of tumor-bearing mice was dramatically prolonged. This study is a pioneer report that demonstrates the use of a biodegradable hydrogel system as an anti-cancer drug delivery system for successful chemoimmunotherapy. It is hoped that, this study can provide a foundation for a rational approach to improve antitumor efficacy of chemoimmunotherapy.