First principles calculations demonstrate the metallization of phosphorene by means of Li doping filling the unoccupied antibonding p_z states. The electron-phonon coupling in the metallic phase is strong enough to eventually lead to a superconducting phase at T_c=17 K for LiP₈ stoichiometry. Using angle-resolved photoemission spectroscopy we confirm that the surface of black phosphorus can be chemically functionalized using Li atoms which donate their 2s electron to the conduction band. The combined theoretical and experimental study demonstrates the semiconductor-metal transition indicating a feasible way to induce a superconducting phase in phosphorene and few-layer black phosphorus.