Abstract: Conventional vaccines, particularly those formulated with aluminum salt adjuvants, are ineffective at inducing cellular immunity, limiting the therapeutic efficacy of tumor vaccines. Herein, using ovalbumin (OVA) as a template, an OVA-loaded manganese phosphate nanovaccine (OVA@MnP) was constructed via a biomineralization strategy. The nanovaccine was systematically characterized by TEM, SEM, and XPS, confirming its well-defined morphology and elemental composition. OVA@MnP exhibited excellent physiological stability and pH-responsive release behavior, enabling triggered antigen and Mn²⁺ release in acidic environments. Cellular uptake and lysosomal escape were confirmed using fluorophore-conjugated OVA@MnP-FITC in DC2.4 cells. Upon cytosolic entry, released Mn²⁺ enhanced cGAS sensitivity, significantly upregulating STING and pSTING levels and potently activating the cGAS-STING pathway. This activation increased bone marrow-derived dendritic cells (BMDCs) maturation by 2.41-fold and elevated the M1/M2 macrophage ratio by 7.54-fold, effectively inducing antigen-specific cellular immune responses. In summary, OVA@MnP represents a promising tumor vaccine candidate, offering a novel strategy for cancer immunotherapy.