The discovery of Na3Bi as a three-dimensional (3D) topological Dirac semimetal (TDS) is reported. TDSs, which can be viewed as the 3D counterpart of graphene, possess 3D Dirac fermions in the bulk, distinct from the 2D Dirac fermions in graphene or the surface states of 3D topological insulators. The electronic structure of Na3Bi was investigated using angle-resolved photoemission spectroscopy (ARPES), revealing linear dispersions along all momentum directions, confirming the presence of 3D Dirac fermions. These fermions are protected by the bulk crystal symmetry, as demonstrated by the robustness of the Dirac fermions against in-situ surface modification. The findings establish Na3Bi as the first model system of 3D TDSs, offering a platform for studying quantum phase transitions between various novel topological states, such as topological insulators, Weyl semimetals, Axion insulators, and topological superconductors. The discovery also highlights the potential for realizing ferromagnetic states through dilute magnetic doping, making Na3Bi an ideal candidate for spintronics applications.The discovery of Na3Bi as a three-dimensional (3D) topological Dirac semimetal (TDS) is reported. TDSs, which can be viewed as the 3D counterpart of graphene, possess 3D Dirac fermions in the bulk, distinct from the 2D Dirac fermions in graphene or the surface states of 3D topological insulators. The electronic structure of Na3Bi was investigated using angle-resolved photoemission spectroscopy (ARPES), revealing linear dispersions along all momentum directions, confirming the presence of 3D Dirac fermions. These fermions are protected by the bulk crystal symmetry, as demonstrated by the robustness of the Dirac fermions against in-situ surface modification. The findings establish Na3Bi as the first model system of 3D TDSs, offering a platform for studying quantum phase transitions between various novel topological states, such as topological insulators, Weyl semimetals, Axion insulators, and topological superconductors. The discovery also highlights the potential for realizing ferromagnetic states through dilute magnetic doping, making Na3Bi an ideal candidate for spintronics applications.