The effective repair of critical-sized bone defects remains a major challenge in bone tissue engineering. While porous tantalum (pTa) scaffolds fabricated by selective laser melting offer excellent mechanical compatibility and structural stability, their inherent biological inertness limits further osteogenic outcomes. This study aimed to develop a functionally enhanced composite scaffold by filling 3D-printed porous tantalum with an alginate hydrogel incorporated with nano-magnesium oxide (MgO) particles (pTa@SA-MgO). The hydrogel acted as a carrier for controlled Mg²⁺ ion release. Comprehensive in vitro and in vivo evaluations revealed that this composite scaffold significantly promoted osteogenic activities, including MC3T3-E1 cell proliferation, migration, adhesion, and differentiation (as evidenced by increased ALP activity, mineralized nodule formation, and upregulation of Runx2, OCN, OPN, and Col I gene expression). Moreover, it modulated the immune microenvironment by enhancing M2 macrophage polarization and suppressing the M1 phenotype. In a rabbit femoral defect model, the pTa@SA-MgO scaffold demonstrated superior bone regeneration compared to controls, with greater bone volume (BV/TV) and enhanced new bone formation. This research successfully integrates a sustained ion release strategy with a high-performance metallic scaffold, providing new insights into the design of bone repair materials with synergistic mechanical support and bioactive functionality.