Similar to graphene, zero bandgap is found in the recently proposed 2D dumbbell C4N (DB C4N), which limits its applications as electronic devices. Surface adsorption effectively modifies the electronic and magnetic properties of the 2D materials. Herein, the adsorption behaviors of 16 metal atoms adsorbed on the DB C4N, including alkali metals, alkaline earth metals, 3d transition metals (TMs), and precious metals, are investigated using the density functional theory. The electronic and magnetic properties of the adatom–C4N systems are systematically explored by analyzing the adsorption energies, the density of states (DOS), the band structures, the charge transfer, the total magnetic moments, the work function, and the dipole moment. It is revealed that the alkali metals (Li, Na), alkaline‐earth metals (Be, Mg), 3d TMs (Ti, V, Cr, Mn, Fe, Co), and precious metals (Ag, Pt, Au) open the zero bandgap of DB C4N via charge transfer between the adatoms and the DB C4N surface. Moreover, the 3d transition and precious metals adatoms induce magnetism in the monolayer C4N. These results qualify the metal‐adsorbed C4N systems as promising candidates for building outstanding electronic and spintronic devices in the future.