The successful application of mRNA vaccines in preventing and treating infectious diseases highlights their potential as therapeutic vaccines for cancer treatment. However, unlike infectious diseases, effective antitumor therapy, particularly for solid tumors, necessitates the activation of more powerful cellular and humoral immunity to achieve clinical efficacy. Here, we report a spleen-targeted mRNA vaccine (Mn@mRNA-LNP) designed to deliver tumor antigen-encoding mRNA and manganese adjuvant (Mn) simultaneously to dendritic cells (DCs) in the spleen. This delivery system promotes DC maturation and surface antigen presentation and stimulates the production of cytotoxic T cells. Additionally, Mn codelivered in the system serves as a safe and effective immune adjuvant, activating the stimulator of interferon genes (STING) signaling pathway and promoting the secretion of type I interferon, further enhancing the antigen-specific T cell responses. Mn@mRNA-LNP effectively inhibits tumor progression in established melanoma and colon tumor models as well as in a model of tumor recurrence after resection. Notably, the combination of Mn@mRNA-LNP with immune checkpoint inhibitors further enhances complete tumor suppression and prolonged the overall survival in mice. Overall, this "All-in-One" mRNA vaccine significantly boosts antitumor immunity responses by improving spleen targeting and immune activation, providing an attractive strategy for the future clinical translation of therapeutic mRNA vaccines.