First-principles calculations were performed to study the adsorption characteristics of 15 different metal atoms on germanene. For the alkali metal adatoms (Li, Na, and K) on germanene, the bonding is approximately ideal ionic and the semimetallic germanene finally becomes metallic with a small band gap opening at the Dirac point. The bonding of alkaline earth metal atoms (Be, Mg, and Ca) to germanene is a mixture of ionic and covalent. The Be and Mg adsorptions lead to semiconducting behavior in germanene, while similar to Li, Na, and K adsorptions, the Ca adsorbed germanene is metallic. For most transition metal adatoms, a strong covalent bonding behavior is found between the adatom and germanene layer, which causes much larger distortions in the germanene lattice. As a result of partially occupied d orbital, the transition metals show also diverse electronic structures when interacting with germanene, such as nonmagnetic metal, nonmagnetic semiconductor, ferromagnetic metal, ferromagnetic semiconductor, and more particularly, ferromagnetic half-metal. In addition, the analysis of the partial density of states indicates that the ferromagnetic property of the obtained transition metal – germanene systems mainly results from the spin-split of the adatom 3d states. The rich electronic and magnetic properties of metal–germanene systems may have potential applications for designing new nanoscale electronic and spintronic devices.